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PROBLEMS  OF  GENETICS 


BY 


WILLIAM   BATESON,  M.A.,  F.R.S. 

DIRECTOR  OF  THE  JOHN  INNES  HO"firiCULTURAL  INSTITUTION,  HON.  FELLOW  OF  ST.  JOHN'S 
COLLEGE,  CAMBRIDGE,  AND  FORMERLY  PROFESSOR  OF  BIOLOGY  IN  THE  UNIVERSITY 


WITH  ILLUSTRATIONS 


NEW  HAVEN:  YALE  UNIVERSITY  PRESS 

LONDON:  HUMPHREY  MILFORD 

OXFORD  UNIVERSITY  PRESS 

MCMXIII 


Copyright,  1913 
By  YALE  UNIVERSITY 

First  printed  August,  1913,  1000  copies 


THE  SILLIMAN  FOUNDATION 

In  the  year  1883  a  legacy  of  about  eighty-five  thousand 
dollars  was  left  to  the  President  and  Fellows  of  Yale  College 
in  the  city  of  New  Haven,  to  be  held  in  trust,  as  a  gift  from  her 
children,  in  memory  of  their  beloved  and  honored  mother,  Mrs. 
Hepsa  Ely  Silliman. 

On  this  foundation  Yale  College  was  requested  and  directed 
to  establish  an  annual  course  of  lectures  designed  to  illustrate 
the  presence  and  providence,  the  wisdom  and  goodness  of  God, 
as  manifested  in  the  natural  and  moral  world.  These  were  to  be 
designated  as  the  Mrs.  Hepsa  Ely  Silliman  Memorial  Lectures. 
It  was  the  belief  of  the  testator  that  any  orderly  presentation 
of  the  facts  of  nature  or  history  contributed  to  the  end  of  this 
foundation  more  effectively  than  any  attempt  to  emphasize  the 
elements  of  doctrine  or  of  creed;  and  he  therefore  provided  that 
lectures  on  dogmatic  or  polemical  theology  should  be  excluded 
from  the  scope  of  this  foundation,  and  that  the  subjects  should  be 
selected  rather  from  the  domains  of  natural  science  and  history, 
giving  special  prominence  to  astronomy,  chemistry,  geology,  and 
anatomy. 

It  was  further  directed  that  each  annual  course  should  be  made 
the  basis  of  a  volume  to  form  part  of  a  series  constituting  a 
memorial  to  Mrs.  Silliman.  The  memorial  fund  came  into  the 
possession  of  the  Corporation  of  Yale  University  in  the  year  1901 ; 
and  the  present  volume  constitutes  the  fifth  of  the  series  of 
memorial  lectures. 


PREFACE 

This  book  gives  the  substance  of  a  series  of  lectures  delivered 
in  Yale  University,  where  I  had  the  privilege  of  holding  the  office 
of  Silliman  Lecturer  in  1907. 

The  delay  in  publication  was  brought  about  by  a  variety  of 
causes. 

Inasmuch  as  the  purpose  of  the  lectures  is  to  discuss  some  of 
the  wider  problems  of  biology  in  the  light  of  knowledge  acquired 
by  Mendelian  methods  of  analysis,  it  was  essential  that  a  fairly 
full  account  of  the  conclusions  established  by  them  should  first 
be  undertaken  and  I  therefore  postponed  the  present  work  till 
a  book  on  Mendel's  Principles  had  been  completed. 

On  attempting  a  more  general  discussion  of  the  bearing  of 
the  phenomena  on  the  theory  of  Evolution,  I  found  myself 
continually  hindered  by  the  consciousness  that  such  treatment 
is  premature,  and  by  doubt  whether  it  were  not  better  that  the 
debate  should  for  the  present  stand  indefinitely  adjourned. 
That  species  have  come  into  existence  by  an  evolutionary 
process  no  one  seriously  doubts;  but  few  who  are  familiar  with 
the  facts  that  genetic  research  has  revealed  are  now  inclined  to 
speculate  as  to  the  manner  by  which  the  process  has  been  ac- 
complished. Our  knowledge  of  the  nature  and  properties  of 
living  things  is  far  too  meagre  to  justify  any  such  attempts. 
Suggestions  of  course  can  be  made:  though,  however,  these 
ideas  may  have  a  stimulating  value  in  the  lecture  room,  they 
look  weak  and  thin  when  set  out  in  print.  The  work  which  may 
one  day  give  them  a  body  has  yet  to  be  done. 

The  development  of  negations  is  always  an  ungrateful  task 
apt  to  be  postponed  for  the  positive  business  of  experiment. 
Such  work  is  happily  now  going  forward  in  most  of  the  centers 
of  scientific  life.  Of  many  of  the  subjects  here  treated  we  already 
know  more  than  we  did  in  1907.  The  delay  in  production  has 
made  it  possible  to  incorporate  these  new  contributions. 

The  book  makes  no  pretence  at  being  a  treatise  and  the 

vii 


Vlll 


PREFACE 


number  of  illustrative  cases  has  been  kept  within  a  moderate 
compass.  A  good  many  of  the  examples  have  been  chosen  from 
American  natural  history,  as  being  appropriate  to  a  book  in- 
tended primarily  for  American  readers.  The  facts  are  largely 
given  on  the  authority  of  others,  and  I  wish  to  express  my 
gratitude  for  the  abundant  assistance  received  from  American 
colleagues,  especially  from  the  staffs  of  the  American  Museum 
in  New  York,  and  of  the  Boston  Museum  of  Natural  History. 
In  connexion  with  the  particular  subjects  personal  acknowledg- 
ments are  made. 

Dr.  F.  M.  Chapman  was  so  good  as  to  supervise  the  prepara- 
tion of  the  coloured  Plate  of  Colaptes,  and  to  authorize  the  loan 
of  the  Plate  representing  the  various  forms  of  Helminthophila, 
which  is  taken  from  his  North  American  Warblers. 

I  am  under  obligation  to  Messrs.  Macmillan  &  Co.,  for  per- 
mission to  reproduce  several  figures  from  Materials  for  the  Study 
of  Variation,  illustrating  subjects  which  I  wished  to  treat  in 
new  associations,  and  to  M.  Leduc  for  leave  to  use  Fig.  9. 

In  conclusion  I  thank  my  friends  in  Yale  for  the  high  honour 
they  did  me  by  their  invitation  to  contribute  to  the  series  of 
Silliman  Lectures,  and  for  much  kindness  received  during  a 
delightful  sojourn  in  that  genial  home  of  learning. 


TABLES  OF  CONTENTS. 

CHAPTER  PAGE 

I.     INTRODUCTORY.    THE   PROBLEM   OF  SPECIES   AND 

VARIETY i 

II.    MERISTIC  PHENOMENA 31 

III.  SEGMENTATION,  ORGANIC  AND  MECHANICAL 60 

IV.  THE  CLASSIFICATION  OF  VARIATION  AND  THE  NATURE 

OF  SUBSTANTIVE  VARIATION 83 

NOTE  TO  CHAPTER  IV 94 

V.    THE  MUTATION  THEORY 97 

NOTE  TO  CHAPTER  V 116 

VI.    VARIATION  AND  LOCALITY 118 

VII.    LOCAL  DIFFERENTIATION — continued.    OVERLAPPING 

FORMS 146 

VIII.     LOCALLY  DIFFERENTIATED  FORMS — continued.    CLI- 
MATIC VARIETIES 164 

IX.    THE  EFFECTS  OF  CHANGED  CONDITIONS 187 

X.    THE  EFFECTS  OF  CHANGED  CONDITIONS — continued. 

THE  CAUSES  OF  GENETIC  VARIATION 212 

XI.    THE    STERILITY    OF    HYBRIDS.    CONCLUDING    RE- 
MARKS    233 

APPENDIX  TO  CHAPTER  X 250 

INDEX 251 


PROBLEMS  OF  GENETICS 


PROBLEMS  OF  GENETICS 


CHAPTER  I 

INTRODUCTORY 

THE  purpose  of  these  lectures  is  to  discuss  some  of  the 
familiar  phenomena  of  biology  in  the  light  of  modern  discoveries. 
In  the  last  decade  of  the  nineteenth  century  many  of  us  per- 
ceived that  if  any  serious  advance  was  to  be  made  with  the  group 
of  problems  generally  spoken  of  as  the  Theory  of  Evolution, 
methods  of  investigation  must  be  devised  and  applied  of  a  kind 
more  direct  and  more  penetrating  than  those  which  after  the 
general  acceptance  of  the  Darwinian  views  had  been  deemed 
adequate.  Such  methods  obviously  were  to  be  found  in  a 
critical  and  exhaustive  study  of  the  facts  of  variation  and  heredity, 
upon  which  all  conceptions  of  evolution  are  based.  To  construct 
a  true  synthetic  theory  of  Evolution  it  was  necessary  that  vari- 
ation and  heredity  instead  of  being  merely  postulated  as  axioms 
should  be  minutely  examined  as  phenomena.  Such  a  study 
Darwin  himself  had  indeed  tentatively  begun,  but  work  of  a 
more  thorough  and  comprehensive  quality  was  required.  In 
the  conventional  view  which  the  orthodoxy  of  the  day  prescribed, 
the  terms  variation  and  heredity  stood  for  processes  so  vague 
and  indefinite  that  no  analytical  investigation  of  them  could  be 
contemplated.  So  soon,  however,  as  systematic  inquiry  into 
the  natural  facts  was  begun  it  was  at  once  found  that  the  ac- 
cepted ideas  of  variation  were  unfounded.  Variation  was  seen 
very  frequently  to  be  a  definite  and  specific  phenomenon,  af- 
fecting different  forms  of  life  in  different  ways,  but  in  all  its 
diversity  showing  manifold  and  often  obvious  indications  of 
regularity.  This  observation  was  not  in  its  essence  novel. 
Several  examples  of  definite  variation  had  been  well  known  to 


2  PROBLEMS  OF  GENETICS 

Darwin  and  others,  but  many,  especially  Darwin  himself  in  his 
later  years,  had  nevertheless  been  disposed  to  depreciate  the 
significance  of  such  facts.  They  consequently  then  lapsed  into 
general  disparagement.  Upon  more  careful  inquiry  the  abun- 
dance of  such  phenomena  proved  to  be  far  greater  than  was 
currently  supposed,  and  a  discussion  of  their  nature  brought 
into  prominence  a  consideration  of  greater  weight,  namely 
that  the  differences  by  which  these  definite  or  discontinuous 
variations  are  constituted  again  and  again  approximate  to  and 
are  comparable  with  the  class  of  differences  by  which  species 
are  distinguished  from  each  other. 

The  interest  of  such  observations  could  no  longer  be  denied. 
The  more  they  were  examined  the  more  apparent  it  became  that 
by  means  of  the  facts  of  variation  a  new  light  was  obtained  on 
the  physiological  composition  and  capabilities  of  living  things. 
Genetics  thus  cease  to  be  merely  a  method  of  investigating 
theories  of  evolution  or  of  the  origin  of  species  but  provide  a 
novel  and  hitherto  untried  instrument  by  which  the  nature  of 
the  living  organism  may  be  explored.  Just  as  in  the  study  of 
non-living  matter  science  began  by  regarding  the  external 
properties  of  weight,  opacity,  colour,  hardness,  mode  of  occur- 
rence, etc.,  noting  only  such  evidences  of  chemical  attributes 
and  powers  as  chance  spontaneously  revealed;  and  much  later 
proceeded  to  the  discovery  that  these  casual  manifestations  of 
chemical  properties,  rightly  interpreted,  afford  a  key  to  the 
intrinsic  nature  of  the  diversity  of  matter,  so  in  biology,  having 
examined  those  features  of  living  things  which  ordinary  obser- 
vations can  perceive,  we  come  at  last  to  realize  that  when  studied 
for  their  own  sake  the  properties  of  living  organisms  in  respect 
of  heredity  and  variation  are  indications  of  their  inner  nature 
and  provide  evidences  of  that  nature  which  can  be  obtained  from 
no  other  source. 

While  such  ideas  were  gradually  forming  in  our  minds,  came 
the  rediscovery  of  Mendel's  work.  Investigations  which  before 
had  only  been  imagined  as  desirable  now  became  easy  to  pursue, 
and  questions  as  to  the  genetic  inter-relations  and  compositions 
of  varieties  can  now  be  definitely  answered.  Without  prejudice 


INTRODUCTORY  3 

to  what  the  future  may  disclose  whether  by  way  of  limitation 
or  extension  of  Mendelian  method,  it  can  be  declared  with 
confidence  and  certainty  that  we  have  now  the  means  of  be- 
ginning an  analysis  of  living  organisms,  and  distinguishing  many 
of  the  units  or  factors  which  essentially  determine  and  cause  the 
development  of  their  several  attributes. 

Briefly  put,  the  essence  of  Mendelism  lies  in  the  discovery 
of  the  existence  of  unit  characters  or  factors.  For  an  account 
of  the  Mendelian  method,  how  it  is  applied  and  what  it  has 
already  accomplished,  reference  must  be  made  to  other  works.1 
With  this  part  of  the  subject  I  shall  assume  a  sufficient  ac- 
quaintance. In  these  lectures  I  have  rather  set  myself  the  task 
of  considering  how  certain  problems  appear  when  viewed  from 
the  standpoint  to  which  the  application  of  these  methods  has 
led  us.  It  is  indeed  somewhat  premature  to  discuss  such  ques- 
tions. The  work  of  Mendelian  analysis  is  progressing  with 
great  rapidity  and  anything  I  can  say  may  very  soon  be  super- 
seded as  out  of  date.  Nevertheless  a  discussion  of  this  kind 
may  be  of  at  least  temporary  service  in  directing  inquiry  to  the 
points  of  special  interest. 

THE  PROBLEM  OF  SPECIES  AND  VARIETY 

Nowhere  does  our  new  knowledge  of  heredity  and  variation 
apply  more  directly  than  to  the  problem  what  is  a  species  and 
what  is  a  variety?  I  cannot  assert  that  we  are  already  in  a 
position  to  answer  this  important  question,  but  as  will  presently 
appear,  our  mode  of  attack  and  the  answers  we  expect  to  re- 
ceive are  not  those  that  were  contemplated  by  our  predecessors. 
If  we  glance  at  the  history  of  the  scientific  conception  of  Species 
we  find  many  signs  that  it  was  not  till  comparatively  recent 
times  that  the  definiteness  of  species  became  a  strict  canon  of 
the  scientific  faith  and  that  attempts  were  made  to  give  precise 
limits  to  that  conception.  When  the  diversity  of  living  things 
began  to  be  accurately  studied  in  the  sixteenth  and  seventeenth 

1  In  Mendel's  Principles  of  Heredity  (Cambridge  University  Press,  1909) 
I  have  dealt  with  this  subject,  giving  an  account  of  the  principal  facts  discovered 
up  to  the  beginning  of  1909. 


4  PROBLEMS  OF  GENETICS 

centuries  names  were  applied  in  the  loosest  fashion,  and  in  giving 
a  name  to  an  animal  or  a  plant  the  naturalists  of  those  times  had 
no  ulterior  intention.  Names  were  bestowed  on  those  creatures 
about  which  the  writer  proposed  to  speak.  When  Gesner  or  Aldro- 
vandi  refer  to  all  the  kinds  of  horses,  unicorns,  dogs,  mermaids, 
etc.,  which  they  had  seen  or  read  of,  giving  to  each  a  descriptive 
name,  they  do  not  mean  to  "elevate"  each  named  kind  to  "spe- 
cific rank";  and  if  anyone  had  asked  them  what  they  meant  by 
a  species,  it  is  practically  certain  that  they  would  have  had  not 
the  slightest  idea  what  the  question  might  imply,  or  any  suspicion 
that  it  raised  a  fundamental  problem  of  nature. 

Spontaneous  generation  being  a  matter  of  daily  observation, 
then  unquestioned,  and  supernatural  events  of  all  kinds  being 
commonly  reported  by  many  witnesses,  transmutation  of  species 
had  no  inherent  improbability.  Matthioli,2  for  instance,  did  not 
expect  to  be  charged  with  heresy  when  he  declared  Stirpium 
mutatio  to  be  of  ordinary  occurrence.  After  giving  instances 
of  induced  modifications  he  wrote,  "Tan turn  enim  in  plantis 
naturae  germanitas  potest,  ut  non  solum  saepe  praedictos 
praestet  effectus,  sed  etiam  ut  alteram  in  alteram  stirpem  facile 
vertat,  ut  cassiam  in  cinnamomum,  sisymbrium  in  mentham, 
triticum  in  lolium,  hordeum  in  avenam,  et  ocymum  in  serpyllum." 

1  do  not  know  who  first  emphasized  the  need  for  a  clear 
understanding  of  the  sense  in  which  the  term  species  is  to  be 
applied.     In  the  second  half  of  the  seventeenth  century  Ray 
shows  some  degree  of  concern  on  this  matter.     In  the  intro- 
duction to    the  Historia  Plantarum,  1686,  he    discusses  some 
of  the  difficulties  and  lays  down  the  principle  that  varieties 
which  can  be  produced  from  the  seed  of  the  same  plant  are  to 
be  regarded  as  belonging  to  one  species,  being,  I  believe,  the 
first  to  suggest  this  definition.     That  new  species  can  come  into 
existence  he  denies  as  inconsistent  with  Genesis  2,  in  which  it  is 
declared  that  God  finished  the  work  of  Creation  in  six  days. 
Nevertheless  he  does  not  wholly  discredit  the  possibility  of  a 
"transmutation"  of  species,  such  that  one  species  may  as  an 
exceptional  occurrence  give  rise  by  seed  to  another  and  nearly 

2  Matthioli  Opera,  Ed.  1598,  p.  8,  originally  published  1565. 


INTRODUCTORY  5 

allied  species.  Of  such  a  phenomenon  he  gives  illustrations  the 
authenticity  of  which  he  says  he  is,  against  his  will,  compelled  to 
admit.  He  adds  that  some  might  doubt  whether  in  the  cases 
quoted  the  two  forms  concerned  are  really  distinct  species,  but 
the  passage  is  none  the  less  of  value  for  it  shews  that  the  con- 
ception of  species  as  being  distinct  unchangeable  entities  was  not 
to  Ray  the  dogma  sacrosanct  and  unquestionable  which  it 
afterwards  became.3 

In  the  beginning  of  the  eighteenth  century  Marchant,4  having 
observed  the  sudden  appearance  of  a  lacinated  variety  of  Mercu- 
rialis,  makes  the  suggestion  that  species  in  general  may  have 
arisen  by  similar  mutations.  Indeed  from  various  passages  it  is 
manifest  that  to  the  authors  of  the  seventeenth  and  early  eigh- 
teenth centuries  species  appeared  simply  as  groups  more  or  less 
definite,  the  boundaries  of  which  it  was  unnecessary  to  determine 
with  great  exactitude.  Such  views  were  in  accord  with  the 
general  scientific  conception  of  the  time.  The  mutability  of 

3  Ray's  instances  relate  to  Kales,  and  in  most  of  these  examples  we  can 
see  that  there  was  no  question  of  mutation  or  transmutation  at  all,  but  that  the 
occurrence  was  due  either  to  mistake  or  to  cross-fertilisation.  Sharrock,  to  whom 
Ray  refers,  was  inclined  to  discredit  stories  of  transmutation,  but  he  has  also  this 
passage  (History  of  the  Propagation  and  Improvement  of  Vegetables  by  the  Concurrence 
of  Art  and  Nature,  Oxford,  1660,  p.  29): 

"  It  is  indeed  growen  to  be  a  great  question,  whether  the  transmutation  of  a 
species  be  possible  either  in  the  vegetable,  Animal,  or  Minerall  Kingdome.  For  the 
possibility  of  it  in  the  vegetable;  I  have  heard  Mr.  Bobart  and  his  Son  often  report  it, 
and  proffer  to  make  oath  that  the  Crocus  and  Gladiolus,  as  likewise  the  Leucoium, 
and  Hyacinths  by  a  long  standing  without  replanting  have  in  his  garden  changed 
from  one  kind  to  the  other:  and  for  satisfaction  about  the  curiosity  in  the  presence 
of  Mr.  Boyle  I  tooke  up  some  bulbs  of  the  very  numericall  roots  whereof  the  re- 
lation was  made,  though  the  alteration  was  perfected  before,  where  we  saw  the 
diverse  bulbs  growing  as  it  were  on  the  same  stoole,  close  together,  but  no  bulb 
hah"  of  the  one  kind,  and  the  other  half  of  the  other:  But  the  changetime  being  past 
it  was  reason  we  should  believe  the  report  of  good  artists  in  matters  of  their  own 
faculty." 

Robert  Sharrock  was  a  fellow  of  New  College,  Oxford.  Both  the  Bobarts  were 
professional  botanists,  the  father  was  author  of  a  Catalogue  of  the  plants  in  the 
Hortus  Medicus  at  Oxford,  and  the  son  was  afterwards  Curator  of  the  Oxford 
Garden. 

*Mem.  Ac.  roy.  des  Sci.  for  1719  (1721),  p.  59- 


6  PROBLEMS  OF  GENETICS 

species  is  for  example  sometimes  likened  (see  for  instance  Shar- 
rock,  loc.  cit.)  to  the  metamorphoses  of  insects,  and  it  is  to  be 
remembered  that  the  search  for  the  Philosopher's  Stone  by  which 
the  transmutation  of  metals  was  to  be  effected  had  only  recently 
fallen  into  discredit  as  a  pursuit. 

The  notion  indeed  of  a  peculiar,  fixed  meaning  to  be  attached 
to  species  as  distinct  from  variety  is  I  think  but  rarely  to  be 
found  categorically  expressed  in  prae-Linnaean  writings. 

But  with  the  appearance  of  the  Systema  Naturae  a  great 
change  supervened.  Linnaeus  was  before  all  a  man  of  order. 
Foreseeing  the  immense  practical  gain  to  science  that  must  come 
from  a  codification  of  nomenclature,  he  invented  such  a  system. 

It  is  not  in  question  that  Linnaeus  did  great  things  for  us  and 
made  Natural  History  a  manageable  and  accessible  collection  of 
facts  instead  of  a  disorderly  heap ;  but  orderliness  of  mind  has 
another  side,  and  inventors  and  interpreters  of  systems  soon  attri- 
bute to  them  a  force  and  a  precision  which  in  fact  they  have  not. 

The  systematist  is  primarily  a  giver  of  names,  as  Ray  with 
his  broader  views  perceived.  Linnaeus  too  in  the  exordium  to 
the  Systema  Naturae  naively  remarks,  that  he  is  setting  out  to 
continue  the  work  which  Adam  began  in  the  Golden  Age,  to  give 
names  to  the  living  creatures.  Naming  however  involves  very 
delicate  processes  of  mind  and  of  logic.  Carried  out  by  the  light 
of  meagre  and  imperfect  knowledge  it  entails  all  the  mischievous 
consequences  of  premature  definition,  and  promotes  facile 
illusions  of  finality.  So  was  it  with  the  Linnaean  system.  An 
interesting  piece  of  biological  history  might  be  written  respecting 
the  growth  and  gradual  hardening  of  the  conception  of  Species. 
To  readers  of  Linnaeus's  own  writings  it  is  well  known  that  his 
views  cannot  be  summarized  in  a  few  words.  Expressed  as  they 
were  at  various  times  during  a  long  life  and  in  various  connexions, 
they  present  those  divers  inconsistencies  which  commonly 
reflect  a  mind  retaining  the  power  of  development.  Nothing 
certainly  could  be  clearer  than  the  often  quoted  declaration  of  the 
Philosophia  Botanica,  "Species  tot  numeramus  quot  diversae 
formae  in  principio  sunt  creatae,"  with  the  associated  passage 
"Varietates  sunt  plantae  ejusdem  speciei  mutatae  a  caussa 


INTRODUCTORY  7 

quacunque  occasional!."  Those  sayings  however  do  not  stand 
alone.  In  several  places,  notably  in  the  famous  dissertation 
on  the  peloric  Linaria  he  explicitly  contemplates  the  possibility 
that  new  species  may  arise  by  crossing,  declaring  nevertheless 
that  he  thinks  such  an  event  to  be  improbable.  In  that  essay 
he  refers  to  Marchant's  observation  on  a  laciniate  Mercurialis, 
but  though  he  states  clearly  that  that  plant  should  only  be 
regarded  as  a  variety  of  the  normal,  he  does  not  express  any 
opinion  that  the  contemporary  genesis  of  new  species  must  be  an 
impossibility.  In  the  later  dissertation  on  Hybrid  Plants  he 
returns  to  the  same  topic.  Again  though  he  states  the  belief 
that  species  cannot  be  generated  by  cross-breedings,  he  treats 
the  subject  not  as  heretical  absurdity  but  as  one  deserving 
respectful  consideration. 

The  significance  of  the  aphorisms  that  precede  the  lectures 
on  the  Natural  Orders  is  not  easy  to  apprehend.  These  are 
expressed  with  the  utmost  formality,  and  we  cannot  doubt  that 
in  them  we  have  Linnaeus's  own  words,  though  for  the  record 
we  are  dependent  on  the  transcripts  of  his  pupils. 

The  text  of  the  first  five  is  as  follows: 

1.  Creator  T.  O.  in  primordio  vestiit  Vegetabile  Medullare 
principiis    constitutivis    diversi    Corticalis    unde    tot    difformia 
individua,  quot  Ordines  Naturales  prognata. 

2.  Classicas  has  (i)  plantas  Omnipotens  miscuit  inter  se, 
unde  tot  Genera  ordinum,  quot  inde  plantae. 

3.  Genericas  has  (2)  miscuit  Natura,  unde  tot  Species  con- 
generes  quot  hodie  existunt. 

4.  Species  has  miscuit  Casus,   unde  totidem  quot  passim 
occurrunt,  Varietates. 

5.  Suadent   haec    (1-4)    Creatoris   leges   a   simplicibus    ad 
Composita. 

Naturae  leges  generationis  in  hybridis. 

Hominis  leges  ex  observatis  a  posteriori. 

I  am  not  clear  as  to  the  parts  assigned  in  the  first  sentence 
respectively  to  the  "Medulla"  and  the  "Cortex"  beyond  that 
Linnaeus  conceived  that  multiformity  was  first  brought  about 
by  diversity  in  the  "Cortex"  The  passage  is  rendered  still 


8  PROBLEMS  OF  GENETICS 

more  obscure  if  read  in  connection  with  the  essay  on  "Generatio 
Ambigena"  where  be  expresses  the  conviction  that  the  Medulla 
is  contributed  by  the  mother,  and  the  Cortex  by  the  father,  both 
in  plants  and  animals.5 

But  however  that  may  be,  he  regards  this  original  diversity 
as  resulting  in  the  constitution  of  the  Natural  Orders,  each  rep- 
resented by  one  individual. 

In  the  second  aphorism  the  Omnipotent  is  represented  as 
creating  the  genera  by  intermixing  the  individual  plantae  classicae, 
or  prototypes  of  the  Natural  Orders. 

The  third  statement  is  the  most  remarkable,  for  in  it  he 
declares  that  Species  were  formed  by  the  act  of  Nature,  who  by 
inter-mixing  the  genera  produced  Species  congener  es,  namely 
species  inside  each  genus,  to  the  number  which  now  exist. 
Lastly,  Chance  or  Accident,  intermixing  the  species,  produced 
as  many  varieties  as  there  are  about  us. 

Linnaeus  thus  evidently  regarded  the  intermixing  of  an 
originally  limited  number  of  types  as  the  sufficient  cause  of 
all  subsequent  diversity,  and  it  is  clear  that  he  draws  an  an- 
tithesis between  Creator,  Natura,  and  Casus,  assigning  to  each 
a  special  part  in  the  operations.  The  acts  resulting  in  the 
formation  of  genera  are  obviously  regarded  as  completed  within 
the  days  of  the  Creation,  but  the  words  do  not  definitely  show 
that  the  parts  played  by  Nature  and  Chance  were  so  limited. 

Recently  also  E.  L.  Greene6  has  called  attention  to  some 
curious  utterances  buried  in  the  Species  Plantarum,  in  which 
Linnaeus  refers  to  intermediate  and  transitional  species,  using 
language  that  even  suggests  evolutionary  proclivities  of  a 
modern  kind,  and  it  is  not  easy  to  interpret  them  otherwise. 

Whatever  Linnaeus  himself  believed  to  be  the  truth,  the 
effect  of  his  writings  was  to  induce  a  conviction  that  the  species 

B  Amoen.  Acad.,  1789,  vol.  6.  I  do  not  know  whether  attention  has  been  called 
to  the  curious  mistake  which  Linnaeus  makes  in  the  course  of  this  argument.  He 
cites  the  differences  between  the  Mule  and  the  Hinny  in  illustration  of  his  thesis, 
pointing  out  that  the  Mule  is  externally  more  like  a  horse  and  the  Hinny  more  like 
an  ass.  This,  he  says,  is  because  the  Mule  has  the  horse  for  a  father,  and  the 
Hinny  the  ass,  thus  inverting  the  actual  facts! 

*Proc.  Washington  Ac.  Sci.,  1909,  XI,  pp.  17-26. 


INTRODUCTORY  9 

of  animals  and  plants  were  immutably  fixed.  Linnaeus  had 
reduced  the  whole  mass  of  names  to  order  and  the  old  fantastical 
transformations  with  the  growth  of  knowledge  had  lapsed  into 
discredit;  the  fixity  of  species  was  taken  for  granted,  but  not 
till  the  overt  proclamation  of  evolutionary  doctrine  by  Lamarck 
do  we  find  the  strenuous  and  passionate  assertions  of  immutability 
characteristic  of  the  first  half  of  the  nineteenth  century. 

It  is  not  to  be  supposed  that  the  champions  of  fixity  were 
unacquainted  with  varietal  differences  and  with  the  problem 
thus  created,  but  in  their  view  these  difficulties  were  apparent 
merely,  and  by  sufficiently  careful  observation  they  supposed 
that  the  critical  and  permanent  distinctions  of  the  true  species 
could  be  discovered,  and  the  impermanent  variations  detected 
and  set  aside. 

This  at  all  events  was  the  opinion  formed  by  the  great  body 
of  naturalists  at  the  end  of  the  eighteenth  and  beginning  of  the 
nineteenth  centuries,  and  to  all  intents  and  purposes  in  spite 
of  the  growth  of  evolutionary  ideas,  it  remains  the  guiding 
principle  of  systematists  to  the  present  day.  There  are  'good 
species'  and  'bad  species'  and  the  systematists  of  Europe  and 
America  spend  most  of  their  time  in  making  and  debating  them. 

In  some  of  its  aspects  the  problem  of  course  confronted  earlier 
naturalists.  Parkinson  for  instance  (1640)  in  introducing  his 
treatment  of  Hieracium  wrote,  "To  set  forth  the  whole  family  of 
the  Hawkew^eedes  in  due  forme  and  order  is  such  a  world  of 
worke  that  I  am  in  much  doubt  of  mine  own  abilitie,  it  having 
lyen  heavie  on  his  shoudiers  that  hath  already  waded  through 
them  ...  for  such  a  multitude  of  varieties  in  forme  pertaining 
to  one  herbe  is  not  to  be  found  againe  in  rerum  natura  as  I 
thinke,"  and  the  same  idea,  that  the  difficulty  lay  rather  in 
man's  imperfect  powers  of  discrimination  than  in  the  nature  of 
the  materials  to  be  discriminated,  is  reflected  in  many  treatises 
early  and  late. 

It  was  however  with  the  great  ouburst  of  scientific  activity 
which  followed  Linnaeus  that  the  difficulty  became  acute. 
Simultaneously  vast  masses  of  new  material  were  being  collected 
from  all  parts  of  the  world  into  the  museums,  and  the  products 


io  PROBLEMS  OF  GENETICS 

of  the  older  countries  were  re-examined  with  a  fresh  zeal  and  on 
a  scale  of  quantity  previously  unattempted.  But  the  problem 
how  to  name  the  forms  and  where  to  draw  lines,  how  much 
should  be  included  under  one  name  and  where  a  new  name  was 
required,  all  this  was  felt,  rather  as  a  cataloguer's  difficulty 
than  as  a  physiological  problem.  And  so  we  still  hear  on  the 
one  hand  of  the  confusion  caused  by  excessive  "splitting"  and 
subdivisions,  and  on  the  other  of  the  uncritical  "lumpers"  who 
associate  together  under  one  name  forms  which  another  collector 
or  observer  would  like  to  see  distinguished. 

In  spite  of  Darwin's  hopes,  the  acceptance  of  his  views  has 
led  to  no  real  improvement — scarcely  indeed  to  any  change  at 
all  in  either  the  practice  or  aims  of  systematists.  In  a  famous 
passage  in  the  Origin  he  confidently  declares  that  when  his 
interpretation  is  generally  adopted  "Systematists  will  be  able 
to  pursue  their  labours  as  at  present;  but  they  will  not  be  in- 
cessantly haunted  by  the  shadowy  doubt  whether  this  or  that 
form  be  a  true  species.  This,  I  feel  sure,  and  I  speak  after 
experience,  will  be  no  slight  relief.  The  endless  disputes  whether 
or  not  some  fifty  species  of  British  brambles  are  good  species 
will  cease."  Those  disputes  nevertheless  proceed  almost  ex- 
actly as  before.  It  is  true  that  biologists  in  general  do  not, 
as  formerly,  participate  in  these  discussions  because  they  have 
abandoned  systematics  altogether;  but  those  who  are  engaged 
in  the  actual  work  of  naming  and  cataloguing  animals  and 
plants  usually  debate  the  old  questions  in  the  old  way.  There 
is  still  the  same  divergence  of  opinion  and  of  practice,  some  in- 
clining to  make  much  of  small  differences,  others  to  neglect 
them. 

Not  only  does  the  work  of  the  sytematists  as  a  whole  proceed 
as  if  Darwin  had  never  written  but  their  attitude  towards  these 
problems  is  but  little  changed.  In  support  of  this  statement  I  may 
refer  to  several  British  Museum  Catalogues,  much  of  the  Biologia 
Centrali- Americana,  Ridgway's  Birds  of  North  America,  the 
Fauna  Hawaiensis,  indeed  to  almost  any  of  the  most  important 
systematic  publications  of  England,  America,  or  any  other 
country.  These  works  are  compiled  by  the  most  proficient 


INTRODUCTORY  n 

systematists  of  all  countries  in  the  several  groups,  but  with 
rare  exceptions  they  show  little  misgiving  as  to  the  fundamental 
reality  of  specific  differences.  That  the  systematists  consider 
the  species-unit  as  of  primary  importance  is  shown  by  the 
fact  that  the  whole  business  of  collection  and  distribution  of 
specimens  is  arranged  with  regard  to  it. 

Almost  always  the  collections  are  arranged  in  such  a  way  that 
the  phenomena  of  variation  are  masked.  Forms  intermediate 
between  two  species  are,  if  possible,  sorted  into  separate  boxes 
under  a  third  specific  name.  If  a  species  is  liable  to  be  constantly 
associated  with  a  mutational  form,  the  mutants  are  picked  out, 
regardless  of  the  circumstances  of  their  origin,  from  the  samples 
among  which  they  were  captured,  and  put  apart  under  a  special 
name.  Only  by  a  minute  study  of  the  original  labels  of  the 
specimens  and  by  redistributing  them  according  to  locality  and 
dates,  can  their  natural  relations  be  traced.  The  published 
accounts  of  these  collections  often  take  no  notice  of  variations, 
others  make  them  the  subject  of  casual  reference.  Very  few 
indeed  treat  them  as  of  much  importance.  From  such  indi- 
cations it  is  surely  evident  that  the  systematists  attach  to  the 
conception  of  species  a  significance  altogether  different  from  that 
which  Darwin  contemplated. 

I  am  well  aware  that  some  very  eminent  systematists  regard 
the  whole  problem  as  solved.  They  hold  as  Darwin  did  that 
specific  diversity  has  no  physiological  foundation  or  causation 
apart  from  fitness,  and  that  species  are  impermanent  groups, 
the  delimitations  of  which  are  ultimately  determined  by  en- 
vironmental exigency  or  "fitness."  The  specific  diversity  of 
living  things  is  thus  regarded  as  being  something  quite  different 
in  nature  from  the  specific  diversity  of  inorganic  substances. 
In  practice  those  who  share  these  opinions  are,  as  might  be  an- 
ticipated, to  be  found  among  the  Mumpers'  rather  than  among 
the  'splitters/  In  their  work,  certainly,  the  Darwinian  theory 
is  actually  followed  as  a  guiding  principle;  unanalysed  inter- 
gradations  of  all  kinds  are  accepted  as  impugning  the  integrity 
of  species;  the  underlying  physiological  problem  is  forgotten, 
and  while  the  product  is  amost  valueless  as  a  contribution  to 


12  PROBLEMS  OF  GENETICS 

biological  research,  I  can  scarcely  suppose  that  it  aids  greatly 
in  the  advances  of  other  branches  of  our  science. 

But  why  is  it  that,  with  these  exceptions,  the  consequences 
of  the  admittedly  general  acceptance  of  a  theory  of  evolution 
are  so  little  reflected  in  the  systematic  treatment  of  living  things? 
Surely  the  reason  is  that  though  the  systematist  may  be  con- 
vinced of  the  general  truth  of  the  evolution  theory  at  large,  he 
is  still  of  opinion  that  species  are  really  distinct  things.  For 
him  there  are  still  'good'  species  and  'bad'  species  and  his  ex- 
perience tells  him  that  the  distinction  between  the  two  is  not 
simply  a  question  of  degree  or  a  matter  of  opinion. 

To  some  it  may  seem  that  this  is  mere  perversity,  a  refusal 
to  see  obvious  truth,  a  manifestation  of  the  spirit  of  the  collector 
rather  than  of  the  naturalist.  But  while  recognising  that  from 
a  magnification  of  the  conception  of  species  the  systematists 
are  occasionally  led  into  absurdity  I  do  not  think  the  grounds 
for  their  belief  have  in  recent  times  been  examined  with  the 
consideration  they  deserve.  The  phenomenon  of  specific 
diversity  is  manifested  to  a  similar  degree  by  living  things  be- 
longing to  all  the  great  groups,  from  the  highest  to  the  lowest, 
Vertebrates,  Invertebrates,  Protozoa,  Vascular  Plants,  Algae, 
and  Bacteria,  all  present  diversities  of  such  a  kind  that  among 
them  the  existence  of  specific  differences  can  on  the  whole  be 
recognised  with  a  similar  degree  of  success  and  with  very  similar 
limitations.  In  all  these  groups  there  are  many  species  quite 
definite  and  unmistakable,  and  others  practically  indefinite. 
The  universal  presence  of  specificity,  as  we  may  call  it,  simi- 
larly limited  and  characterised,  is  one  of  its  most  remarkable 
features.  Not  only  is  this  specificity  thus  universally  present 
among  the  different  forms  of  life,  but  it  manifests  itself  in  respect 
of  the  most  diverse  characteristics  which  living  things  display. 
Species  may  thus  be  distinguished  by  peculiarities  of  form,  of 
number,  of  geometrical  arrangement,  of  chemical  constitution 
and  properties,  of  sexual  differentiation,  of  development,  and  of 
many  other  properties.  In  any  one  or  in  several  of  these  features 
together,  species  may  be  found  distinguished  from  other  species. 
It  is  also  to  be  observed  that  the  definiteness  of  these  distinctions 


INTRODUCTORY  13 

has  no  essential  dependence  on  the  nature  of  the  characteristic 
which  manifests  them.  It  is  for  example  sometimes  said  that 
colour-distinctions  are  of  small  systematic  importance,  but  every 
systematist  is  familiar  with  examples  (like  that  of  the  wild  species 
of  Callus)  in  which  colours  though  complex,  show  very  little 
variation.  On  the  other  hand  features  of  structure,  sexual  dif- 
ferentiation, and  other  attributes  which  by  our  standards  are 
estimated  as  essential,  may  be  declared  to  show  much  variation 
or  little,  not  according  to  any  principle  which  can  be  detected, 
but  simply  as  the  attention  happens  to  be  applied  to  one  species 
or  group  of  species,  or  to  another.  In  many  groups  of  animals  and 
plants  observers  have  hit  upon  characters  which  were  for  a  time 
thought  to  be  finally  diagnostic  of  species.  The  Lepidoptera  and 
Diptera  for  instance,  have  been  re-classified  according  to  their 
neura tion .  Through  a  considerable  range  of  forms  determinations 
may  be  easily  made  on  these  characters,  but  as  is  now  well 
known,  neura  tion  is  no  more  immune  from  variation  than  any 
other  feature  of  organisation,  and  in  some  species  great  varia- 
bility is  the  rule.  Again  it  was  once  believed  by  some  that  the 
genitalia  of  the  Lepidoptera  provided  a  basis  of  final  determination 
—  with  a  similar  sequel.  In  some  groups,  for  example  the  Lycae- 
nidae,  or  the  Hesperidae,  there  are  forms  almost  or  quite  in- 
distinguishable on  external  examination,  but  a  glance  at  the 
genitalia  suffices  to  distinguish  numerous  species,  while  on  the 
contrary  among  Pieridae  a  great  range  of  species  show  scarcely 
any  difference  in  these  respects:  and  again  in  occasional  species 
the  genitalia  show  very  considerable  variations. 

The  proposition  that  animals  and  plants  are  on  the  whole 
divisible  into  definite  and  recognisable  species  is  an  approxi- 
mation to  the  truth.  Such  a  statement  is  readily  defensible, 
whereas  to  assert  the  contrary  would  be  palpably  absurd.  For 
example,  a  very  competent  authority  lately  wrote:  "In  the 
whole  Lepidopterous  fauna  of  England  there  is  no  species  of 
really  uncertain  limits."  7  Others  may  be  disposed  to  make 
certain  reservations,  but  such  exceptions  would  be  so  few  as 
scarcely  to  impair  the  validity  of  the  general  statement.  The 

7  J.  W.  Tutt,  in  Ent.  Rec.,  1909,  XXI,  p.  185. 


H  PROBLEMS  OF  GENETICS 

declaration  might  be  extended  to  other  orders  and  other 
lands. 

We  know,  of  course,  that  the  phenomenon  of  specific  diversity 
is  complicated  by  local  differentiation:  that,  in  general,  forms 
which  cannot  disperse  themselves  freely  exhibit  a  multitude  of 
local  races,  and  that  of  these  some  are  obviously  adaptative, 
and  that  a  few  even  owe  their  peculiarity  to  direct  envitonmental 
effects.  Every  systematist  also  is  perfectly  aware  that  in  dealing 
with  collections  from  little  explored  countries  the  occurrence 
of  polymorphism  or  even  of  sporadic  variation  may  make  the 
practical  business  of  distinguishing  the  species  difficult  and 
perhaps  for  the  time  impossible;  still,  conceding  that  a  great 
part  of  the  diversity  is  due  to  geographical  differentiation,  and 
that  some  is  sporadic  variation,  our  experience  of  our  own  floras 
and  faunas  encourages  the  belief  that  if  we  were  thoroughly 
familiar  with  these  exotic  productions  it  would  usually  be 
possible  to  assign  their  specific  limitations  with  an  approach 
to  certainty. 

For  apart  from  any  question  of  the  justice  of  these  wider 
inferences,  if  we  examine  the  phenomenon  of  specificity  as  it 
appears  in  those  examples  which  are  nearest  to  hand,  surely  we 
find  signs  in  plenty  that  specific  distinction  is  no  mere  consequence 
of  Natural  Selection.  The  strength  of  this  proposition  has 
lain  mainly  in  the  appeal  to  ignorance.  Steadily  with  the  growth 
of  knowledge  has  its  cogency  diminished,  and  such  a  belief 
could  only  have  been  formulated  at  a  time  when  the  facts  of 
variation  were  unknown. 

In  Darwin's  time  no  serious  attempt  had  been  made  to  ex- 
amine the  manifestations  of  variability.  A  vast  assemblage  of 
miscellaneous  facts  could  formerly  be  adduced  as  seemingly 
comparable  illustrations  of  the  phenomenon  "Variation." 
Time  has  shown  this  mass  of  evidence  to  be  capable  of  analysis. 
When  first  promulgated  it  produced  the  impression  that  varia- 
bility was  a  phenomenon  generally  distributed  amongst  living 
things  in  such  a  way  that  the  specific  divisions  must  be  arbitrary. 
When  this  variability  is  sorted  out,  and  is  seen  to  be  in  part  a 
result  of  hybridisation,  in  part  a  consequence  of  the  persistence 


INTRODUCTORY  15 

of  hybrids  by  parthenogenetic  reproduction,  a  polymorphism 
due  to  the  continued  presence  of  individuals  representing  various 
combinations  of  Mendelian  allelomorphs,  partly  also  the  tran- 
sient effect  of  alteration  in  external  circumstances,  we  see  how 
cautious  we  must  be  in  drawing  inferences  as  to  the  indefiniteness 
of  specific  limits  from  a  bare  knowledge  that  intermediates  exist. 
Conversely,  from  the  accident  of  collocation  or  from  a  mislead- 
ing resemblance  in  features  we  deem  essential,  forms  genetically 
distinct  are  often  confounded  together,  and  thus  the  divergence 
of  such  forms  in  their  other  features,  which  we  declare  to  be 
non-essential,  passes  as  an  example  of  variation.  Lastly,  and 
this  is  perhaps  the  most  fertile  of  all  the  sources  of  confusion, 
the  impression  of  the  indefiniteness  of  species  is  created  by  the 
existence  of  numerous  local  forms,  isolated  geographically  from 
each  other,  forms  whose  differences  may  be  referable  to  any  one 
of  the  categories  I  have  enumerated. 

The  advance  has  been  from  many  sides.  Something  has 
come  from  the  work  of  systematists,  something  from  cultural 
experiments,  something  from  the  direct  study  of  variation  as  it 
appears  in  nature,  but  progress  is  especially  due  to  experimental 
investigation  of  heredity.  From  all  these  lines  of  inquiry  we 
get  the  same  answer;  that  what  the  naturalists  of  fifty  years 
ago  regarded  as  variation  is  not  one  phenomenon  but  many, 
and  that  what  they  would  have  adduced  as  evidence  against 
the  definiteness  of  species  may  not  in  fact  be  capable  of  this 
construction  at  all. 

If  we  may  once  more  introduce  a  physical  analogy,  the  dis- 
tinctions with  which  the  systematic  naturalist  is  concerned  in 
the  study  of  living  things  are  as  multifarious  as  those  by  which 
chemists  were  confronted  in  the  early  days  of  their  science. 
Diversities  due  to  mechanical  mixtures,  to  allotropy,  to  differences 
of  temperature  and  pressure,  or  to  degree  of  hydration,  had  all 
to  be  severally  distinguished  before  the  essential  diversity  due 
to  variety  of  chemical  constitution  stood  out  clearly,  and  I 
surmise  that  not  till  a  stricter  analysis  of  the  diversities  of  animals 
and  plants  has  been  made  on  a  comprehensive  scale,  shall  we 
be  in  a  position  to  declare  with  any  confidence  whether  there  is 


16  PROBLEMS  OF  GENETICS 

or  is  not  a  natural  and  physiological  distinction  between  species 
and  variety. 

As  I  have  said  above,  it  is  in  the  cases  nearest  to  hand  that 
the  problem  may  be  most  effectively  studied.  Comparison 
between  forms  from  dissimilar  situations  contributes  something; 
but  it  is  by  a  close  examination  of  the  behaviour,  especially  the 
genetic  behaviour,  of  familiar  species  when  living  in  the  presence 
of  their  nearest  allies  that  the  most  direct  light  on  the  problem 
is  to  be  obtained.  I  cannot  understand  the  attitude  of  those  who, 
contemplating  such  facts  as  this  examination  elicits,  can  com- 
placently declare  that  specific  difference  is  a  mere  question  of 
degree.  With  the  spread  of  evolutionary  ideas  to  speak  much 
of  the  fixity  of  species  has  become  unfashionable,  and  yet  how 
striking  and  inscrutable  are  the  manifestations  of  that  fixity! 

Consider  the  group  of  species  composing  the  agrestis  section 
of  the  genus  Veronica,  namely  Tournefortii,  agrestis,  and 
polita. 

These  three  grow  side  by  side  in  my  garden,  as  they  do  in 
suitable  situations  over  a  vast  area  of  the  temperate  regions. 
I  have  for  years  noticed  them  with  some  care  and  become  familiar 
with  their  distinctions  and  resemblances.  Never  is  there  any 
real  doubt  as  to  the  identity  of  any  plant.  The  species  show 
some  variability,  but  I  have  never  seen  one  which  assumed  any 
of  the  distinguishing  features  of  the  others.  A  glance  at  the 
fruits  decides  at  once  to  which  species  a  plant  belongs.  I  find 
it  impossible  to  believe  that  the  fixity  of  these  distinctions  is 
directly  dependent  on  their  value  as  aids  in  the  struggle  for 
existence.  The  mode  of  existence  of  the  three  forms  in  so  far 
as  we  can  tell  is  closely  similar.  By  whatever  standard  we  reckon 
systematic  affinity  I  suppose  we  shall  agree  that  these  species 
come  very  near  indeed  to  each  other.  Bentham  even  takes 
the  view  that  polita  is  a  mere  variety  of  agrestis. 

Now  in  such  cases  as  this  it  has  been  argued  that  the  specific 
features  of  the  several  types  have  been  separately  developed 
in  as  many  distinct  localities,  and  that  their  present  association 
is  due  to  subsequent  redistribution.  Of  these  Veronicas  indeed 
we  know  that  one,  Tournefortii  (=Buxbaumii)  is  as  a  matter  of  fact 


INTRODUCTORY  17 

a  recent  introduction  from  the  east.8  But  this  course  of  argument 
leads  to  still  further  difficulties.  For  if  it  is  true  that  the  peculiari- 
ties of  the  several  species  have  been  perfected  and  preserved  on 
account  of  their  survival-value  to  their  possessors,  it  follows 
that  there  must  be  many  ways  of  attaining  the  same  result. 
But  since  sufficient  adaptation  may  be  ensured  in  so  many  ways, 
the  disappearance  of  the  common  parent  of  these  forms  is  dif- 
ficult to  understand.  Obviously  it  must  have  been  a  plant 
very  similar  in  general  construction  to  its  modern  representatives. 
Like  them  it  must  have  been  an  annual  weed,  with  an  organisation 
conformable  to  that  mode  of  life.  Why  then,  after  having  been 
duly  perfected  for  that  existence  should  it  have  been  entirely 
superseded  in  favour  of  a  number  of  other  distinct  contrivances 
for  doing  the  same  thing,  and — if  a  gradual  transition  be  predi- 
cated— not  only  by  them,  but  by  each  intermediate  stage 
between  them  and  the  original  progenitor?  Surely  the  obvious 
inference  from  such  facts  is  that  the  burden  cast  upon  the  theory 
of  gradual  selection  is  far  greater  than  it  can  bear;  that  adapta- 
tion is  not  in  practice  a  very  close  fit,  and  that  the  distinctions 
between  these  several  species  of  Veronica  have  not  arisen  on 
account  of  their  survival-value  but  rather  because  none  of  their 
diversities  was  so  damaging  as  to  lead  to  the  extermination  of 
its  possessor.  When  we  see  these  various  Veronicas  each  rigidly 
reproducing  its  parental  type,  all  comfortably  surviving  in 
competition  with  each  other,  are  we  not  forced  to  the  conclusion 
that  tolerance  has  as  much  to  do  with  the  diversity  of  species 
as  the  stringency  of  Selection?  Certainly  these  species  owe  their 
continued  existence  to  the  fact  that  they  are  each  good  enough 
to  live,  but  how  shall  we  refer  the  distinctions  between  them  di- 
rectly or  indirectly  to  the  determination  of  Natural  Selection? 

SE.  Lehmann  (Bull.  I'Herb.  Boissier,  Sen  2,  VIII,  1908,  p.  229)  has  published 
an  admirable  paper  on  the  interrelationships  of  these  species  and  has  instituted 
cultural  experiments  which  will  probably  much  elucidate  the  nature  of  their  specific 
distinctness.  As  regards  the  existence  of  intermediate  forms  he  comes  to  the  conclu- 
sion that  two  only  can  be  so  regarded.  The  first  was  described  by  Kuntze  from 
specimens  found  on  a  flower-pot  on  board  a  Caspian  steamer,  from  which  Leh- 
mann proposes  the  new  specific  name  Siaretensis.  This  comes  between  polita  and 
filifarmis,  a  close  ally  of  Tournefortii.  The  other,  which  combines  some  of  the 
features  of  both  polita  and  Tournefortii,  was  found  in  the  province  of  Asterabad. 
3 


i8  PROBLEMS  OF  GENETICS 

The  control  of  Selection  is  loose  while  the  conformity  to 
specific  distinction  is  often  very  strict  and  precise,  and  no  less 
so  even  when  several  closely  related  species  co-exist  in  the  same 
area  and  in  the  same  circumstances. 

The  theory  of  Selection  fails  at  exactly  the  point  where  it 
was  devised  to  help:  Specific  distinction. 

Let  us  examine  a  somewhat  different  set  of  facts  in  the  case 
of  another  pair  of  nearly  allied  species  Lychnis  diurna  and  ves- 
pertina.  The  two  plants  have  much  in  common.  Both  are 
dioecious  perennials,  with  somewhat  similar  flowers,  the  one 
crimson,  the  other  white.  Each  however  has  its  peculiarities 
which  are  discernible  in  almost  any  part  of  its  structure,  whether 
flower,  leaf,  fruit  or  seed,  distinctions  which  would  enable  a 
person  thoroughly  familiar  with  the  plants  to  determine  at  once 
from  which  species  even  a  small  piece  had  been  taken.  There 
is  so  much  resemblance  however  as  readily  to  support  the  surmise 
that  the  two  were  mere  varieties  of  one  species.  Bentham, 
following  Linnaeus,  in  fact  actually  makes  this  suggestion, 
with  what  propriety  we  will  afterwards  consider.  Now  this  case 
is  typical  of  many.  The  two  forms  have  a  wide  distribution, 
occurring  sometimes  separately,  sometimes  in  juxtaposition. 
L.  diurna  is  a  plant  of  hedgerows  and  sheltered  situations.  L. 
vesper tina  is  common  in  fields  and  open  spaces,  where  diurna 
is  hardly  ever  found;  but  not  rarely  vespertina  occurs  in  associa- 
tion with  diurna  in  the  places  which  that  plant  frequents.  In 
this  case  I  do  not  doubt  that  we  have  to  do  with  organisms  of 
somewhat  different  aptitudes.  That  L.  vespertina  has  powers 
which  diurna  has  not  is  shown  very  clearly  by  the  fact  that 
diurna  is  sometimes  entirely  absent  from  areas  where  vespertina 
can  abound.9  But  in  order  to  understand  the  true  genetic 
relations  of  the  two  plants  to  each  other  it  is  necessary  to  observe 
their  behaviour  when  they  meet  as  they  not  unfrequently  do. 

g  In  Cambridgeshire  for  example  vespertina  is  common  but  diurna  is  absent. 
Whether  this  absence  is  connected  with  the  general  presence  of  chalk  I  cannot  say. 
When  introduced  artificially  diurna  establishes  itself,  for  a  time  at  least,  without  any 
apparent  difficulty  and  occasionally  escapes  from  the  garden  on  to  the  neighbouring 
roadside. 


INTRODUCTORY  19 

If  the  Lychnis  population  of  such  a  locality  be  examined  it  will 
be  found  to  consist  of  many  undoubted  and  unmodified  diurna, 
a  number — sometimes  few,  sometimes  many — of  similarly 
unmodified  vespertina,  and  an  uncertain  but  usually  rather  small 
proportion  of  plants  obviously  hybrids  between  the  two.  How 
is  it  possible  to  reconcile  these  facts  with  the  view  that  specific 
distinction  has  no  natural  basis  apart  from  environmental 
exigency? 

Darwinian  orthodoxy  suggests  that  by  a  gradual  process  of 
Natural  Selection  either  one  of  these  two  types  was  evolved  from 
the  other,  or  both  from  a  third  type.  I  cannot  imagine  that 
anyone  familiar  with  the  facts  would  propose  the  first  hypothesis 
in  the  case  of  Lychnis,  nor  can  I  conceive  of  any  process,  whether 
gradual  or  sudden,  by  which  diurna  could  have  come  out  of 
vespertina,  or  vespertina  out  of  diurna.  Both  however  may  no 
doubt  have  been  derived  from  some  original  third  type.  It  is 
conceivable  that  Lychnis  macrocarpa  of  Boissier,  a  native  of 
Southern  Spain  and  Morocco,  may  be  this  original  form.  This 
species  is  said  to  combine  a  white  flower  (like  that  of  L.  ves- 
pertina), with  capsule- teeth  rolled  back  (like  those  of  diurna).10 
But  whatever  the  common  progenitor  may  have  been,  if  we  are 
to  believe  that  these  two  species  have  been  evolved  from  it  by 
a  gradual  process  of  Natural  Selection  based  on  adaptation, 
enormous  assumptions  must  be  made  regarding  the  special  fitness 
of  these  two  forms  and  the  special  unfitness  of  the  common 
parent,  and  these  assumptions  must  be  specially  invoked  and 
repeated  for  each  several  feature  of  structure  or  habits  distin- 
guishing the  three  forms. 

Why,  if  the  common  parent  was  strong  enough  to  live  to  give 
rise  to  these  two  species,  is  it  either  altogether  lost  now,  or  at 
least  absent  from  the  whole  of  Northern  Europe?  Its  two 
putative  descendants,  though  so  distinct  from  each  other,  are, 
as  we  have  seen,  able  often  to  occupy  the  same  ground.  If 
they  were  gradually  derived  from  a  common  progenitor — 
necessarily  very  like  themselves — can  we  believe  that  this  original 

10  Conceivably  however  it  may  be  a  segregated  combination.  For  an  account 
of  this  plant  see  Boissier,  Voy.  Bot.  Midi  de  I'Espagne,  1839,  II,  722. 


20  PROBLEMS  OF  GENETICS 

form  should  always,  in  all  the  diversities  of  soil  and  situation 
which  they  inhabit,  be  unable  to  exist?  Some  one  may  fancy 
that  the  hybrids  which  are  found  in  the  situations  occupied  by 
both  forms  are  this  original  parental  species.  But  nothing  can 
be  more  certain  than  that  these  plants  are  simply  heterozygous 
combinations  made  by  the  union  of  gametes  bearing  the  characters 
of  diurna  and  vespertina. n  For  they  may  be  reproduced  exactly 
in  FI  or  in  later  generations  of  that  cross  when  it  is  artificially 
made ;  when  bred  from  their  families  exhibit  palpable  phenomena 
of  segregation  more  or  less  complex;  and  usually,  if  perhaps  not 
always,  they  are  partially  sterile.12  In  a  locality  on  the  Norfolk 
coast  that  I  know  well,  there  is  a  strip  of  rough  ground  chiefly 
sand-bank,  which  runs  along  the  shore.  This  ground  is  full  of 
vespertina.  Not  a  hundred  yards  inland  is  a  lane  containing 
diurna,  and  among  the  vespertina  on  the  sand-bank  are  always 
some  of  the  hybrid  form,  doubtless  the  result  of  fertilisation 
from  the  heighbouring  diurna  population.  Seed  saved  from  these 
hybrids  gave  vespertina  and  hybrids  again,  having  obviously  been 
fertilised  by  other  vespertina  or  by  other  hybrids,  and  I  have 
no  doubt  that  such  hybrid  plants  if  fertilised  by  diurna  would 
have  shown  some  diurna  offspring.  The  absence  of  diurna 
in  such  localities  may  fairly  be  construed  as  an  indication  that 
diurna  is  there  at  a  real  disadvantage  in  the  competition  for 
life. 

But  if,  admitting  this,  we  proceed  to  consider  how  the  special 
aptitude  of  vespertina  is  constituted,  or  what  it  is  that  puts 
diurna  at  a  disadvantage,  we  find  ourselves  quite  unable  to 
show  the  slightest  connexion  between  the  success  of  one  or  the 

11 A  discussion  of  this  subject  with  references  to  literature  is  given  by  Rolf e, 
in  an  excellent  paper  on  "  Hybridisation  viewed  from  the  standpoint  of  Systematic 
Botany"  (Jour.  R.  Hort.  Soc.,  XXIV,  1900,  p.  197).  He  concludes:  "The  simple 
fact  is  that  the  two  plants  (L.  diurna  and  vespertina)  are  thoroughly  distinct  in 
numerous  particulars,  and  affect  such  different  habitats  that  in  some  localities 
one  or  the  other  of  them  is  completely  wanting.  But  when  their  stations  are 
adjacent  they  hybridise  together  very  readily,  and  it  is  here  that  these  intermediate 
forms  occur  which  have  puzzled  botanists  so  much."  The  same  paper  contains 
valuable  information  concerning  several  cognate  illustrations. 

12  In  only  two  cases  have  I  seen  such  plants  (both  females)  completely  sterile. 


INTRODUCTORY  21 

failure  of  the  other  on  the  one  hand,  and  the  specific  character* 
istics  which  distinguish  the  two  forms  on  the  other.  The  or- 
thodox Selectionist  would,  as  usual,  appeal  to  ignorance.  We 
ask  what  can  vespertina  gain  by  its  white  flowers,  its  more  lan- 
ceolate leaves,  its  grey  seeds,  its  almost  erect  capsule- teeth, 
its  longer  fruits,  which  diurna  loses  by  reason  of  its  red  flowers, 
more  ovate  leaves,  dark  seeds,  capsule-teeth  rolled  back,  and 
shorter  fruits?  We  are  told  that  each  of  these  things  may 
affect  the  viability  of  their  possessors.  We  cannot  assert  that 
this  is  untrue,  but  we  should  like  to  have  evidence  that  it  is  true. 
The  same  problem  confronts  us  in  thousands  upon  thousands 
of  examples,  and  as  time  goes  on  we  begin  to  feel  that  speculative 
appeals  to  ignorance,  though  dialectically  admissible,  provide 
an  insufficient  basis  for  a  proposition  which,  if  granted,  is  to 
become  the  foundation  of  a  vast  scheme  of  positive  construction. 

One  thing  must  be  abundantly  clear  to  all,  that  to  treat  two 
forms  so  profoundly  different  as  one,  because  intermediates  of 
unknown  nature  can  be  shown  to  exist  between  them,  is  a  mere 
shirking  of  the  difficulties,  and  this  course  indeed  creates  artificial 
obstacles  in  the  way  of  those  who  are  seeking  to  discover  the 
origin  of  organic  diversity. 

In  the  enthusiasm  with  which  evolutionary  ideas  were  re- 
ceived the  specificity  of  living  things  was  almost  forgotten. 
The  exactitude  with  which  the  members  of  a  species  so  often 
conform  in  the  diagnostic,  specific  features  passed  out  of  account; 
and  the  scientific  world  by  dwelling  with  a  constant  emphasis 
on  the  fact  of  variability,  persuaded  itself  readily  that  spe- 
cies had  after  all  been  a  mere  figment  of  the  human  mind. 
Without  presuming  to  declare  what  future  research  only  can 
reveal,  I  anticipate  that,  when  variation  has  been  properly 
examined  and  the  several  kinds  of  variability  have  been  suc- 
cessfully distinguished  according  to  their  respective  natures, 
the  result  will  render  the  natural  definiteness  of  species  increas- 
inlgy  apparent.  Formerly  in  such  a  case  as  that  of  the  two 
Lychnis  species,  the  series  of  "intermediates"  was  taken  to  be  a 
palpable  proof  that  vespertina  "graded"  to  diurna.  It  is  this 
fact,  doubtless,  upon  which  Bentham  would  have  relied  in  sug- 


22  PROBLEMS  OF  GENETICS 

gesting  that  both  may  be  one  species.13  Genetic  tests,  though  as 
yet  imperfectly  applied,  make  it  almost  certain  that  these  inter- 
grading  forms  are  not  in  any  true  sense  variations  from  either 
species  in  the  direction  of  the  other,  but  combinations  of  elements 
derived  from  both. 

The  points  in  which  very  closely  allied  species  are  distin- 
guished from  each  other  may  be  found  in  the  most  diverse 
features  of  their  organisation.  Sometimes  specific  difference 
is  to  be  seen  in  a  character  which  we  can  believe  to  be  important 
in  the  struggle,  but  at  least  as  often  it  is  some  little  detail  that 
we  cannot  but  regard  as  trivial  which  suffices  to  differentiate 
the  two  species.  Even  when  the  diagnostic  point  is  of  such  a 
nature  that  we  can  imagine  it  to  make  a  serious  difference  in  the 
economy  we  are  absolutely  at  a  loss  to  suggest  why  this  feature 
should  be  a  necessity  to  species  A  and  unnecessary  to  species  B 
its  nearest  ally.  The  house  sparrow  (Passer  domesticus)  is  in 
general  structure  very  like  the  tree  sparrow  (P.  montanus). 
They  differ  in  small  points  of  colour.  For  instance  montanus 
has  a  black  patch  on  the  cheek  which  is  absent  in  domesticus. 
The  presence  in  the  one  species  and  the  absence  in  the  other 
are  equally  definite,  and  in  both  cases  we  are  equally  unable  to 
suggest  any  consideration  of  utility  in  relation  to  these  features. 
The  two  species  are  distinguished  also  by  a  characteristic  that 
may  well  be  supposed  to  be  of  great  significance.  In  domesticus 
the  two  sexes  are  strongly  differentiated,  the  cock  being  more 
ornate  than  the  hen.  On  the  other  hand  the  two  sexes  in  mon- 
tanus are  alike,  and,  if  we  take  a  standard  from  domesticus,  we 
may  fairly  say  that  in  montanus  the  hen  has  the  colouration  of 
the  male.  It  is  not  unreasonable  to  suppose  that  such  a  dis- 
tinction may  betoken  some  great  difference  in  physiological 
economy,  but  the  economical  significance  of  this  perhaps  im- 
portant distinction  is  just  as  unaccountable  as  that  of  the  seem- 
ingly trivial  but  equally  diagnostic  colour-point. 

I  have  spoken  of  the  fixed  characteristics  of  the  two  species. 

13  As  is  well  known,  in  an  even  more  notorious  example,  he  proposed  to  unite 
Primula  vulgaris,  P.  elatior,  and  P.  acaulis,  similarly  relying  on  the  existence  of 
"intermediates,"  which  we  now  well  know  to  be  mongrels  between  the  species. 


INTRODUCTORY  23 

If  we  turn  to  a  very  different  feature,  their  respective  liability 
to  albinistic  variation,  we  find  ourselves  in  precisely  similar 
difficulty.  Passer  domesticus  is  a  species  in  which  individuals 
more  or  less  pied  occur  with  especial  frequency,  but  in  P.  mon- 
tanus  such  variation  is  extremely  rare  if  it  occurs  at  all.  The 
writer  of  the  section  on  Birds  in  the  Royal  Natural  History 
(III.,  1894-5,  P-  393)  calls  attention  to  this  fact  and  remarks 
that  in  that  species  he  knows  no  such  instance. 

The  two  species  therefore,  apart  from  any  differences  that  we 
can  suppose  to  be  related  to  their  respective  habits,  are  charac- 
terised by  small  fixed  distinctions  in  colour-markings,  by  a 
striking  difference  in  secondary  sexual  characters,  and  by  a 
difference  in  variability.  In  all  these  respects  we  can  form 
no  surmise  as  to  any  economic  reason  why  the  one  species 
should  be  differentiated  in  the  one  way  and  the  other  in  the  other 
way,  and  I  believe  it  is  mere  self-deception  which  suggests  the 
hope  that  with  fuller  knowledge  reasons  of  this  nature  would  be 
discovered. 

The  two  common  British  wasps,  Vespa  vulgaris  and  Vespa 
germanica,  are  another  pair  of  species  closely  allied  although 
sharply  distinguished,  which  suggest  similar  reflexions.  Both 
usually  make  subterranean  nests  but  of  somewhat  different 
materials.  F.  vulgaris  uses  rotten  wood  from  which  the  nest 
derives  a  characteristic  yellow  colour,  while  F.  germanica  scrapes, 
off  the  weathered  surfaces  of  palings  and  other  exposed  timber, 
material  which  is  converted  into  the  grey  walls  of  the  nest.  The 
stalk  by  which  the  nest  is  suspended  (usually  to  a  root)  in  the 
case  of  germanica  passes  freely  through  a  hole  in  the  external 
envelope,  but  vulgaris  unites  this  external  wall  solidly  to  the 
stalk.  In  bodily  appearance  and  structure  the  two  species  are 
so  much  alike  that  they  have  often  been  confounded  even  by 
naturalists,  and  to  the  untrained  observer  they  are  quite  indis- 
tinguishable. There  are  nevertheless  small  points  of  difference 
which  almost  though  not  quite  always  suffice  to  distinguish  the 
two  forms.  For  example  the  yellow  part  of  the  sinus  of  the  eyes 
is  emarginate  in  vulgaris  but  not  emarginate  in  germanica.  V. 
vulgaris  often  has  black  spots  on  the  tibiae  while  in  germanica  the 


24  PROBLEMS  OF  GENETICS 

tibiae  are  usually  plain  yellow.  In  both  species  there  is  a  hori- 
zontal yellow  stripe  on  the  thorax,  but  whereas  in  vulgaris  this  is 
a  plain  narrow  stripe,  it  is  in  germanica  enlarged  downwards  in 
the  middle.  These  and  other  apparently  trivial  details  of  colour- 
ation, though  not  absolutely  constant,  are  yet  so  nearly  constant 
that  irregularities  in  these  respects  are  quite  exceptional.  Lastly 
the  genitalia  of  the  males,  though  not  very  different,  present 
small  structural  points  of  distinction  which  are  enough  to  distin- 
guish the  two  species  at  a  glance.14 

In  considering  the  meaning  of  the  distinctions  between  these 
two  wasps  we  meet  the  old  problem  illustrated  by  the  Sparrows. 
The  two  species  have  somewhat  different  habits  of  life  and  we 
should  readily  expect  to  find  differences  of  bodily  organisation 
corresponding  with  the  differences  of  habits.  But  is  that  what 
we  do  find?  Surely  not.  To  suppose  that  there  is  a  corre- 
spondence between  the  little  points  of  colour  and  structure  which 
we  see  and  the  respective  modes  of  life  of  the  two  species  is 
perfectly  gratuitous.  We  have  no  inkling  of  the  nature  of  such 
a  correspondence,  how  it  can  be  constituted,  or  in  what  it  may 
consist. 

Is  it  not  time  to  abandon  these  fanciful  expectations  which 
are  never  realised?  Everywhere  both  among  animals  and  plants 
does  the  problem  of  specific  difference  reiterate  itself  in  the  same 
form.  In  view  of  such  facts  as  I  have  related  and  might  indefi- 
nitely multiply,  the  fixity  of  specific  characters  cannot  readily 
be  held  to  be  a  measure  of  their  economic  importance  to  their 
possessors.  The  incidence  of  specific  fixity  is  arbitrary  and 
capricious,  sometimes  lighting  on  a  feature  or  a  property  which 
can  be  supposed  to  matter  much,  but  as  often  is  it  attached  to  the 
most  trifling  of  superficial  peculiarities. 

The  incidence  of  variability  is  no  less  paradoxical,  and  without 
investigation  of  the  particular  case  no  one  can  say  what  will  be 

"For  an  account  of  the  distinctions  between  Vespa  vulgaris  and  germanica 
see  Ch.  Janet,  Etudes  sur  les  Fourmis,  les  GuZpes  el  les  Abeilles,  ne,  Note.  Sur 
Vespa  germanica  et  V.  vulgaris.  Limoges  (Ducourtieux),  1895;  and  R.  du  Buysson, 
Monographic  des  Gudpes,  Ann.  Soc.  Ent.  France,  1903,  Vol.  LXXII,  p.  603,  PI. 
VIII. 


INTRODUCTORY  25 

found  to  show  much  or  little  variability.  The  very  charac- 
teristic which  in  one  species  may  exhibit  extreme  variability 
may  in  an  allied  species  show  extreme  constancy.  Illustrations 
will  occur  to  any  naturalist,  but  nowhere  is  this  truth  more 
strikingly  presented  than  in  the  British  Noctuid  Moths.  Many 
are  so  variable  that,  in  the  common  phrase,  "scarcely  two  can 
be  found  alike,"  while  others  show  comparatively  slight  variation. 
It  need  scarcely  be  remarked  that,  in  the  instances  I  have  in 
mind,  the  evidence  of  great  variability  is  in  no  way  due  to  the 
abundance  with  which  the  particular  species  occurs,  for  common 
species  may  show  constancy,  and  less  abundant  species  may  show 
great  variability.  The  polymorphism  seems  to  be  now  at  least 
a  general  property  of  the  variable  species,  as  the  fixity  is  a 
property  of  the  fixed  species.  In  illustration  I  may  refer  to  the 
following  examples. 

Dianthoecia  capsincola  is  a  common  and  widely  distributed 
moth  which  feeds  on  Lychnis.  It  shows  little  variation.  Dian- 
thoecia carpophaga  is  another  species  which  feeds  chiefly  on 
Silene.  Its  habits  are  very  similar  to  those  of  capsincola.  Like 
that  species  it  has  a  wide  geographical  range  and  is  abundant 
in  its  localities,  but  in  contrast  to  the  fixity  of  capsincola,  car- 
pophaga exhibits  a  complex  series  of  varieties.  A  gratis  suffusa 
(=  ypsilon)  is  a  moth  widely  spread  through  the  southern  half 
of  England.  It  is  very  constant  in  colour  and  markings.  Ag- 
rotis  segetum  and  tritici  are  excessively  variable  both  in  ground 
colour  and  markings,  being  found  in  an  immense  profusion  of 
dissimilar  forms  throughout  their  distribution.  Of  these  and 
several  other  species  of  A  gratis  there  are  many  named  varieties, 
some  of  which  have  by  various  writers  been  regarded  as  speci- 
fically distinct.  Of  the  genus  Nocttia  many  species  (e.  g.  f estiva) 
show  a  similar  polymorphism,  but  N.  triangulum,  though  showing 
some  variation  in  certain  respects,  is  usually  very  constant  to 
its  type,  and  the  same  is  true  of  N.  umbrosa. 

In  several  species  of  Taeniocampa,  especially  instabilisf  the 
multiplicity  of  forms  is  extreme,  while  cruda  (=  pulverulenta) 
is  a  comparatively  constant  species.  The  genus  Plusia  contains 
a  number  of  constant  species,  but  in  Plusia  interrogationis  we 


26  PROBLEMS  OF  GENETICS 

meet  the  fact  that  the  central  silvery  mark  undergoes  endless 
variation.  "Truly  no  two  are  alike,"  says  Mr.  Tutt,  "and  to 
look  down  a  long  series  of  interrogationis  is  something  like  looking 
at  a  series  of  Chinese  characters."  In  contrast  to  this  we  have 
the  fact  that  in  Plusia  gamma  the  very  similar  silvery  mark  is 
by  no  means  variable. 

I  have  taken  this  series  of  cases  from  the  Noctuid  moths, 
but  it  would  be  as  easy  to  illustrate  the  same  proposition  from 
the  Geometridae  or  the  Micro-Lepidoptera.15  I  have  a  long 
series  of  Peronea  cristana,ior  example,  which  was  given  tome  by 
Mr.  W.  H .  B .  Fletcher,  of  Bognor .  All  were  beaten  out  of  the  same 
hedge,  and  their  polymorphism  is  such  that  no  one  unaccustomed 
to  such  examples  could  suppose  that  they  belonged  to  a  single 
species.  Another  common  form,  P.  schalleriana,  which  lives  in 
similar  circumstances,  exhibits  comparatively  slight  variability. 

It  should  be  expressly  noted  that  the  variation  of  which  I  am 
speaking  is  a  genuine  polymorphism.  Several  of  the  species 
enumerated  exhibit  also  geographical  variation,  possessing  defi- 
nite and  often  strikingly  distinct  races  peculiar  to  certain 
localities;  but  apart  from  the  existence  of  such  local  differen- 
tiation, stands  out  the  fact  upon  which  I  would  lay  stress,  that 
some  species  are  excessively  variable  while  others  are  by  com- 
parison constant,  in  circumstances  that  we  may  fairly  regard 
as  comparable. 

This  fact  is  difficult  to  reconcile  with  the  conventional  view 
that  specific  type  is  directly  determined  by  Natural  Selection 

15  The  statements  made  above  are  for  the  most  part  taken  from  Barrett,  C.  G., 
Lepidoptera  of  the  British  Islands,  and  from  Tutt,  J.  W.,  The  British  Noctuae  and 
their  Varieties.  The  reader  who  is  unfamilar  with  the  amazing  polymorphism 
exhibited  by  some  of  these  moths  should  if  possible  take  an  opportunity  of  looking 
over  a  long  series  in  a  collection,  or,  if  that  be  impossible,  refer  to  the  admirable 
coloured  plates  published  by  Barrett.  It  may  not  be  superfluous  to  observe  that 
plenty  of  similar  examples  are  known  in  other  countries.  For  instance  Plotheia 
frontalis,  a  Noctuid  which  often  abounds  in  Ceylon,  shows  an  equally  bewildering 
wealth  of  forms.  If  a  dozen  specimens  of  such  a  species  were  to  be  brought  home 
from  some  little  known  country,  each  individual  would  almost  certainly  be  described 
as  the  type  of  a  distinct  species.  (See  the  coloured  plate  published  by  Sir  G.  Hamp- 
son,  Cat.  Brit.  Mus.,  Heterocera,  Vol.  IX.) 


INTRODUCTORY  27 

and  that  the  precision  with  which  a  species  conforms  to  its 
pattern  is  an  indication  of  the  closeness  of  that  control.  Anyone 
familiar  with  the  characteristics  of  Moths  will  agree  that  the 
Noctuids,  Geometrids  and  Tortricids  are  creatures  whose  existence 
depends  in  some  degree  on  the  success  with  which  they  can  escape 
detection  by  their  enemies  in  the  imaginal  state.  We  are  there- 
fore not  surprised  to  find  that  some  species  of  these  orders 
exhibit  definite  geographical  variation  in  conformity  with  the 
character  of  the  ground,  which  may  reasonably  be  supposed  to 
aid  in  their  protection.  If  this  were  all,  there  would  be  nothing  to 
cause  surprise.  We  might  even  be  disposed  to  allow  that  varia- 
bility might  contribute  to  the  perpetuation  of  animals  so  situ- 
ated, on  the  principle  that  among  a  variety  of  surroundings 
some  would  probably  be  in  harmony  with  the  objects  on  which 
they  rest.  But  we  cannot  admit  the  plausibility  of  an  argument 
which  demands  on  the  one  hand  that  the  extreme  precision  with 
which  species  A  adheres  in  the  minutest  details  of  its  colour  and 
pattern  to  a  certain  type  shall  be  ascribed  to  the  protective  fitness 
of  those  details,  and  on  the  other  hand  that  the  abundant  varia- 
bility of  species  B  shall  be  ascribed  to  the  same  determination. 
If  it  is  absolutely  necessary  for  A  to  conform  to  one  type  how 
comes  it  that  B  may  range  through  some  twenty  distinct  forms, 
any  two  of  which  differ  more  from  each  other  than  the  regular 
species  of  many  other  genera?  The  only  reply  I  can  conceive 
is  a  suggestion  that  there  may  be  some  circumstance  which 
differentiates  the  various  classes  of  cases,  that  the  exigencies 
of  the  fixed  species  may  be  different  from  those  of  the  variable. 
Those  who  make  such  appeals  to  ignorance  do  not  always  perhaps 
realise  whither  this  course  of  reasoning  may  lead.  If  admissible 
here  the  same  argument  would  lead  us  to  suggest  that  because 
albino  moles  have  for  an  indefinite  period  occurred  on  a  certain 
land  near  Bath  there  may  be  something  in  the  soil  or  in  the 
conditions  of  life  near  Bath  which  requires  a  proportion  of  albinos 
in  its  mole  population.  Or  again,  because  the  butterfly  Thais 
rumina  in  one  locality,  Digne  in  the  south  of  France,  has  a  per- 
centage of  individuals  of  the  variety  Honoratii  (with  certain 
normally  yellow  spots  on  the  hind  wing  coloured  bright  red) 


28  PROBLEMS  OF  GENETICS 

and  nowhere  else  throughout  its  distribution,  that  therefore  we 
may  suggest  that  there  is  some  difference  in  the  condition  of 
life  at  Digne  which  makes  the  continuance  of  Honoratii  there 
possible  and  beneficial. 

A  polymorphism  offering  a  parallel  to  that  of  the  variable 
moths  is  afforded  by  the  breeding  plumage  of  the  Ruff,  the 
male  of  Machetes  pugnax.  The  variety  of  plumage  which  these 
cocks  exhibit  is  such  that  the  statement  that  no  two  can  be 
found  alike  is  only  a  venial  exaggeration.  Newton  remarks16 
"that  all  this  wonderful  'show*  is  the  consequence  of  the  poly- 
gamous habit  of  the  Ruff  can  scarcely  be  doubtful";  but  even  if 
it  be  conceded  that  the  great  external  differentiation  of  the 
cocks  may  be  a  result  of  sexual  selection,  the  problem  of  their 
polymorphism  remains  unsolved,  for,  as  we  are  well  aware, 
polygamy  is  not  usually  associated  with  polymorphism  of  the 
male.  The  Black  Cock  (Tetrao  tetrix),  for  example,  is  as  polyga- 
mous as  the  Ruff,  but  in  that  and  countless  other  cases,  both 
sexes  are  constant  to  one  type  of  plumage. 

When  we  thus  compare  the  polymorphism  of  one  species  with 
the  fixity  of  another,  and  attempt  to  determine  the  causes  which 
have  led  to  these  extraordinary  contrasts,  two  distinct  lines  of 
argument  are  open  to  us.  We  may  ascribe  the  difference  either 
to  causes  external  to  the  organisms,  primarily,  that  is  to  say, 
to  a  difference  in  the  exigencies  of  Adaptation  under  Natural 
Selection;  or  on  the  other  hand  we  may  conceive  the  difference 
as  due  to  innate  distinctions  in  the  chemical  and  physiological 
constitutions  of  the  fixed  and  the  variable  respectively.  There 
is  truth  undoubtedly  in  both  conceptions.  If  the  mole  were 
physiologically  incapable  of  producing  an  albino  that  variety 
would  not  have  come  into  being,  and  if  the  albino  were  totally 
incapable  of  getting  its  living  it  would  not  be  able  to  hold  its 

18  Diet,  of  Birds,  p.  800.  It  would  be  interesting  and  profitable  to  attempt  in 
a  long  series  of  Ruffs  to  determine  the  Mendelian  factors  which  by  their  combinations 
give  rise  to  this  complex  assemblage  of  varietal  forms.  A  few  such  factors  both  of 
colour  and  pattern  can  be  at  once  distinguished,  and  it  is  noticeable  that  some  of 
the  resulting  types  of  barring,  spangling  and  penciling  show  a  perceptible  corre- 
spondence with  some  of  the  types  of  colouration  found  in  the  breeds  of  domestic 
fowls. 


INTRODUCTORY  29 

own.  Were  Plotheia  frontalis  constructed  on  a  chemical  plan 
which  admitted  of  no  variation,  the  countless  varieties  would 
not  have  been  produced ;  and  if  one  of  its  varieties  had  an  over- 
whelming success  out  of  all  proportion  to  that  of  the  rest,  then 
the  species  would  soon  become  monomorphic  again.  We 
cannot  declare  that  Natural  Selection  has  no  part  in  the  deter- 
mination of  fixity  or  variability;  nevertheless  looking  at  the  whole 
mass  of  fact  which  a  study  of  the  incidence  of  variation  provides, 
I  incline  to  the  view  that  the  variability  of  polymorphic  forms 
should  be  regarded  rather  as  a  thing  tolerated  than  as  an  element 
contributing  directly  to  their  chances  of  life;  and  on  the  other 
hand  that  the  fixity  of  the  monomorphic  forms  should  be  looked 
upon  not  so  much  as  a  proof  that  Natural  Selection  controls 
them  with  a  greater  stringency,  but  rather  as  evidence  of  a  natural 
and  intrinsic  stability  of  chemical  constitution. 

Compare  the  condition  of  a  variable  form  like  the  male 
Ruff  (or  in  a  less  degree  the  Red  Grouse  in  both  its  sexes)  with 
that  of  the  common  Pheasant  which  is  comparatively  constant. 
In  the  Pheasant  no  doubt  variations  do  occur  as  in  other  wild 
birds,  but  apart  from  the  effects  of  mongrelisation  the  species 
is  unquestionably  uniform.  Could  it  seriously  be  proposed 
that  we  should  regard  the  constancy  of  the  pheasant's  plumage 
in  this  country  as  depending  on  the  special  fitness  of  that  type 
of  colouration?  Even  if  the  pheasant  be  not  an  alien  in  Western 
Europe,  it  has  certainly  been  protected  for  centuries,  and  for  a 
considerable  period  has  existed  in  a  state  of  semi-domestication. 
Such  conditions  should  give  good  opportunity  for  polymorphism 
to  be  produced.  In  some  coverts  various  aberrations  do  of 
course  occur  and  persist,  yet  there  is  nothing  indicative  of  a 
general  relaxation  of  the  fixity  of  the  specific  type,  and  the  pheas- 
ant remains  substantially  a  fixed  species.1  The  common  pheasant 
(Phasianus  colchicus)  even  shows  little  of  that  disposition  to 

1  Howard  Saunders  (Illust.  Manual  of  British  Birds,  1899,  p.  499)  states  that 
there  is  evidence  that  the  pheasant  had  become  naturalized  in  the  south  of  England 
before  the  Norman  invasion.  He  adds,  "little,  if  any,  deviation  from  the  typical 
P.  colchicus  took  place  up  to  the  end  of  last  century,  when  the  introduction  of 
the  Chinese  Ring-necked  P.  torquatus  commenced,  which  has  left  almost  indelible 
marks,  especially  with  regard  to  the  characteristic  white  collar." 


3o  PROBLEMS  OF  GENETICS 

form  local  races  which  appears  in  the  species  of  Further  India. 
Are  we  not  then  on  safer  ground  in  regarding  the  fixity  of  our 
species  as  a  property  inherent  in  its  own  nature  and  consti- 
tution? Just  as  in  ages  of  domestication  no  rose  has  ever  given 
off  a  blue  variety  so  has  the  pheasant  never  broken  out  into  the 
polymorphism  of  the  Ruff. 

As  soon  as  it  is  realised  how  largely  the  phenomena  of  vari- 
ation and  stability  must  be  an  index  of  the  internal  constitution 
of  organisms,  and  not  mere  consequences  of  their  relations  to 
the  outer  world,  such  phenomena  acquire  a  new  and  more 
profound  significance. 


CHAPTER  II 

MERISTIC   PHENOMENA 

Twenty  years  ago  in  describing  the  facts  of  Variation,  argu- 
ment was  necessary  to  show  that  these  phenomena  had  a  special 
value  in  the  sciences  of  Zoology  and  Botany.  This  value  is 
now  universally  understood  and  appreciated.  In  spite  however 
of  the  general  attention  devoted  to  the  study  of  Variation,  and 
the  accumulation  of  material  bearing  on  the  problem,  no  satis- 
factory or  searching  classification  of  the  phenomena  is  possible. 
The  reason  for  this  failure  is  that  a  real  classification  must  pre- 
suppose knowledge  of  the  chemistry  and  physics  of  living  things 
which  at  present  is  quite  beyond  our  reach. 

It  is  however  becoming  probable  that  if  more  knowledge  of 
the  chemical  and  physical  structure  of  organisms  is  to  be  at- 
tained, the  clue  will  be  found  through  Genetics,  and  thus  that 
even  in  the  uncoordinated  accumulation  of  facts  of  Variation 
we  are  providing  the  means  of  analysis  applicable  not  only  to 
them,  but  to  the  problems  of  normality  also. 

The  only  classification  that  we  can  yet  institute  with  any 
confidence  among  th  phenomena  of  Variation  is  that  which 
distinguishes  on  the  one  hand  variations  in  the  processes  of 
division  from  variations  in  the  nature  of  the  substances  divided. 

Variations  in  the  processes  of  division  are  most  often  made 
apparent  by  a  change  in  the  number  of  the  parts,  and  are  therefore 
called  M eristic  Variations,  while  the  changes  in  actual  composition 
of  material  are  spoken  of  as  Substantive  Variations.  The  Me- 
ristic  Variations  form  on  the  whole  a  natural  and  fairly  well 
defined  group,  but  the  Substantive  Variations  are  obviously 
a  heterogeneous  assemblage. 

Though  this  distinction  does  not  go  very  far,  it  is  useful, 
and  in  all  probability  fundamental.  It  is  of  value  inasmuch  as 
it  brings  into  prominence  the  distinct  and  peculiar  part  which 

31 


32  PROBLEMS  OF  GENETICS 

the  process  of  division,  or,  more  generally,  repetition  of  parts, 
plays  in  the  constitution  of  the  forms  of  living  things. 

That  there  may  be  a  real  independence  between  the  M eristic 
and  the  Substantive  phenomena  is  evident  from  the  fact  both 
that  M  eristic  changes  may  occur  without  Substantive  Variation, 
and  that  the  substances  composing  an  organism  may  change 
without  any  perceptible  alteration  in  its  meristic  structure. 
When  the  distinction  between  these  two  classes  of  phenomena 
is  perceived  it  will  be  realised  that  the  study  of  genetics  has  on 
the  one  hand  a  physical,  or  perhaps  more  strictly  a  mechanical 
aspect,  which  relates  to  the  manner  in  which  material  is  divided 
and  distributed;  and  also  a  chemical  aspect,  which  relates  to 
the  constitution  of  the  materials  themselves.  Somewhat  as 
the  philosophers  of  the  seventeenth  and  eighteenth  centuries 
were  awaiting  both  a  chemical  and  a  mechanical  discovery  which 
should  serve  as  a  key  to  the  problems  of  unorganised  matter, 
so  have  biologists  been  awaiting  two  several  clues.  In  Mendelian 
analysis  we  have  now,  it  is  true,  something  comparable  with  the 
clue  of  chemistry,  but  there  is  still  little  prospect  of  penetrating 
the  obscurity  which  envelops  the  mechanical  aspect  of  our  phe- 
nomena. To  make  clear  the  application  of  the  terms  chemical 
and  mechanical  to  the  problem  of  Genetics  the  nature  of  that 
problem  must  be  more  fully  described.  In  its  most  concrete 
form  this  problem  is  expressed  in  the  question,  how  does  a  cell 
divide?  If  the  organism  is  unicellular,  and  the  single  cell  is 
the  whole  body,  then  the  process  of  heredity  is  accomplished 
in  the  single  operation  of  cell-division.  Similarly  in  animals  and 
plants  whose  bodies  are  made  up  of  many  cells,  the  whole  process 
of  heredity  is  accomplished  in  the  cell-divisions  by  which  the 
germ-cells  are  formed.  When  therefore  we  see  a  cell  dividing, 
we  are  witnessing  the  process  by  which  the  form  and  the  proper- 
ties of  the  daughter-cells  are  determined. 

Now  this  process  has  the  two  aspects  which  I  have  called 
mechanical  and  chemical.  The  term  "  Entwicklungsmechanik " 
has  familiarised  us  with  the  application  of  the  word  mechanics 
to  these  processes,  but  on  reflexion  it  will  be  seen  that  this  com- 
prehensive term  includes  two  sorts  of  events  which  are  sometimes 


MERISTIC  PHENOMENA  33 

readily  distinguishable.  There  is  the  event  by  which  the  cell 
divides,  and  the  event  by  which  the  two  halves  or  their  descend- 
ants are  or  may  be  differentiated.  It  is  common  knowledge  that 
in  some  cell-divisions  two  similar  halves,  indistinguishable  in 
appearance,  properties,  and  subsequent  fate,  may  be  produced, 
while  in  other  divisions  daughter-cells  with  distinct  properties 
and  powers  are  formed.  We  cannot  imagine  but  that  in  the 
first  case,  when  the  resulting  cells  are  idenfical,  the  division 
is  a  mechanical  process  by  which  the  mother-cell  is  simply 
cut  in  two;  while  in  order  that  two  differentiated  halves  may  be 
produced,  some  event  must  have  taken  place  by  which  a  chemical 
distinction  between  the  two  halves  is  effected.1  In  any  ordinary 
Mendelian  case  we  have  a  clear  proof  that  such  a  chemical  dif- 
ference may  be  established  between  germ-cells.  The  facts  of 
colour-inheritance  for  instance  prove  that  germ-cells,  otherwise 
identical,  may  be  formed  possessing  the  chromogen-f actor  which 
is  necessary  to  the  formation  of  colour  in  the  flowers,  or  destitute 
of  that  factor.  Similarly  the  germ-cells  may  possess  the  ferment 
which,  by  its  action  on  the  chromogenic  substance,  produces 
the  colour,  or  they  may  be  without  that  ferment.  The  same 
line  of  argument  applied  to  a  great  range  of  cases.  Nevertheless, 
though  differences  in  chemical  properties  are  often  thus  consti- 
tuted by  cell-divisions,  and  though  we  are  thus  able  to  make  a 
quasi-chemical  analysis  of  the  individual  by  determining  and 
enumerating  these  properties,  yet  it  is  evident  that  the  dis- 
tribution of  these  factors  is  not  itself  a  chemical  process.  This 
is  proved  by  the  fact  that  similar  divisions  may  be  effected  be- 
tween halves  which  are  exactly  alike,  and  also  by  the  fact  that 
the  numbers  in  which  the  various  types  of  germ-cells  are  formed 
negative  any  suggestion  of  valency  between  them.  The  recog- 
nition of  the  unit-factors  may  lead — indeed  must  lead — to  great 
advances  in  chemical  physiology  which  without  that  clue  would 
have  been  impossible,  but  in  causation  the  chemical  phenomena 
of  heredity  must  be  regarded  as  secondary  to  the  physical  or 

1  In  saying  this  we  make  no  assumption  as  to  the  particular  cell-division  at 
which  differentiation  occurs.  This  may  be  one  of  the  maturation-divisions,  or  it 
may  perhaps  be  much  earlier. 


34  PROBLEMS  OF  GENETICS 

mechanical  phenomena  by  which  the  cells  and  their  constituents 
are  divided  and  separated.  When  therefore  we  speak  of  the 
essential  phenomena  of  heredity  we  mean  the  mechanics  of  divi- 
sion, especially,  though  not,  as  we  shall  see,  exclusively,  of  cell- 
division  ;  and  in  the  relation  between  the  two  halves  of  the  divid- 
ing cell  we  have  the  problem  presented  in  what  seems  to  be  its 
simplest  form. 

In  attempting  to  form  some  conception  of  the  processes  by 
which  bodily  characteristics  are  transmitted,  or — to  avoid  that 
confusing  metaphor  of  "transmission" — how  it  comes  about 
that  the  offspring  can  grow  to  resemble  its  parent,  continuity  of 
the  germ-substance  which  in  some  animals  is  a  visible  phenom- 
enon,2 gives  at  least  apparent  help.  An  egg  for  example  on  be- 
coming adult  develops  in  certain  parts  a  particular  pigment. 
The  eggs  of  that  adult  when  they  reach  the  appropriate  age  de- 
velop the  same  pigment.  We  have  no  clear  picture  of  the 
mechanism  by  which  this  process  is  effected,  but  when  we  realise 
that  the  pigment  results  from  the  interaction  of  certain  sub- 
stances, and  that  since  all  the  eggs  are  in  reality  pieces  of  the  same 
material,  it  seems,  unless  we  inquire  closely,  not  unnatural  that 
the  several  pieces  of  the  material  should  exhibit  the  same  colours 
at  the  same  periods  of  their  development.  The  continuity  of 
the  material  of  the  germs  suggests  that  there  is  a  continuity 
of  the  materials  from  which  the  pigment  is  formed,  and  that 
thus  an  actual  bit  of  those  substances  passess  into  each  egg 
ready  at  the  appropriate  moment  to  generate  the  pigment. 
The  argument  thus  outlined  applies  to  all  substantive  character- 
istics. In  each  case  we  can  imagine,  if  we  will,  the  appearance 
of  that  characteristic  as  due  to  the  contribution  of  its  rudiment 
from  the  germ  tissues. 

When  we  consider  more  critically  it  becomes  evident  that 
the  aid  given  by  this  mental  picture  is  of  very  doubtful  reality, 
for  even  if  it  were  true  that  any  predestined  particle  actually 
corresponding  with  the  pigment-forming  materials  is  definitely 

2  From  the  recent  discoveries  of  Envin  Baur  we  are  led  to  surmise  that  in  the 
flowering  plants  the  sub-epidermal  layer,  or  some  of  its  elements,  may  legitimately 
be  regarded  as  a  similar  germ-substance,  continuous  in  Weismann's  sense. 


MERISTIC  PHENOMENA  35 

passed  on  from  germ  to  germ,  yet  the  power  of  increase  which 
must  be  attributed  to  it  remains  so  incomprehensible  that  the 
mystery  is  hardly  at  all  illuminated. 

When  however  we  pass  from  the  substantive  to  the  meristic 
characters,  the  conception  that  the  character  depends  on  the 
possession  by  the  germ  of  a  particle  of  a  specific  material  becomes 
even  less  plausible.  Hardly  by  any  effort  of  imagination  can  we 
see  any  way  by  which  the  division  of  the  vertebral  column  into  x 
segments  or  into  y  segments,  or  of  a  Medusa  into  4  segments  or 
into  6,  can  be  determined  by  the  possession  or  by  the  want  of  a 
material  particle.  The  distinction  must  surely  be  of  a  different 
order.  If  we  are  to  look  for  a  physical  analogy  at  all  we  should 
rather  be  led  to  suppose  that  these  differences  in  segmental 
numbers  corresponded  with  changes  in  the  amplitude  or  number 
of  dividing  waves  than  with  any  change  in  the  substance  or 
material  divided. 

PHENOMENA  OF  DIVISION 

I  have  said  that  in  the  division  of  a  cell  we  seem  to  see  the 
problem  in  its  simplest  form,  but  it  is  important  to  observe  that 
the  problem  of  division  may  be  presented  by  the  bodies  of  animals 
and  plants  in  forms  which  are  independent  of  the  divisions  be- 
tween cells.  The  existence  of  pattern  implies  a  repetition  of 
parts,  and  repetition  of  parts  when  developed  in  a  material 
originally  homogeneous  can  only  be  created  by  division.  Cell- 
division  is  probably  only  a  special  case  of  a  process  similar  to 
that  by  which  the  pattern  of  the  skeleton  is  laid  down  in  a  uni- 
cellular body  such  as  that  of  a  Radiolarian  or  Foraminiferan. 
Attempts  have  lately  been  made  to  apply  mathematical  treat- 
ment to  problems  of  biology.  It  has  sometimes  seemed  to 
me  that  it  is  in  the  geometrical  phenomena  of  life  that  the  most 
hopeful  field  for  the  introduction  of  mathematics  will  be  found. 
If  anyone  will  compare  one  of  our  animal  patterns,  say  hat  of  a 
zebra's  hide,  with  patterns  known  to  be  of  purely  mechanical 
production,  he  will  need  no  argument  to  convince  him  that  there 
must  be  an  essential  similarity  between  the  processes  by  which 
the  two  kinds  of  patterns  were  made  and  that  parts  at  least  of 


36  PROBLEMS  OF  GENETICS 

the  analysis  applicable  to  the  mechanical  patterns  are  applicable 
to  the  zebra  stripes  also.  Patterns  mechanically  produced  are 
of  many  and  very  diverse  kinds.  One  of  the  most  familiar 
examples,  and  one  presenting  some  especially  striking  analogies 
to  organic  patterns,  is  that  provided  by  the  ripples  of  a  mackerel 
sky,  or  those  made  in  a  flat  sandy  beach  by  the  wind  or  the  ebbing 
tide.  With  a  little  search  we  can  find  among  the  ripple-marks,  and 
in  other  patterns  produced  by  simple  physical  means,  the  closest 
parallels  to  all  the  phenomena  of  striping  as  we  see  them  in  our 
animals.  The  forking  of  the  stripes,  the  differentiation  of  two 
"faces,"  the  deflections  round  the  limbs  and  so  forth,  which  in  the 
body  we  know  to  be  phenomena  of  division,  are  common  both  to 
the  mechanical  and  the  animal  patterns.  We  cannot  tell  what  in 
the  zebra  corresponds  to  the  wind  or  the  flow  of  the  current,  but 
we  can  perceive  that  in  the  distribution  of  the  pigments,  that  is 
to  say,  of  the  chromogen-substances  or  of  the  ferments  which 
act  upon  them,  a  rhythmical  disturbance  has  been  set  up  which 
has  produced  the  pattern  we  see;  and  I  think  we  are  entitled  to 
the  inference  that  in  the  formation  of  patterns  in  animals  and 
plants  mechanical  forces  are  operating  which  ought  to  be,  and 
will  prove  to  be,  capable  of  mathematical  analysis.  The  com- 
parison between  the  striping  of  a  living  organism  and  the  sand- 
ripples  will  serve  us  yet  a  little  farther,  for  a  pattern  may  either 
be  formed  by  actual  cell-divisions,  and  the  distribution  of  dif- 
ferentiation coincidently  determined,  or — as  visibly  in  the  pig- 
mentation of  many  animal  and  plant  tissues — the  pattern  may 
be  laid  down  and  the  pigment  (for  example)  distributed  through 
a  tissue  across  or  independently  of  the  cell-divisions  of  the  tissue. 
Our  tissues  therefore  are  like  a  beach  composed  of  sands  of 
different  kinds,  and  different  kinds  of  sands  may  show  distinct 
and  interpenetrating  ripples.  When  the  essential  analogy  be- 
tween these  various  classes  of  phenomena  is  perceived,  no  one 
will  be  astonished  at,  or  reluctant  to  admit,  the  reality  of  dis- 
continuity in  Variation,  and  if  we  are  as  far  as  ever  from  knowing 
the  actual  causation  of  pattern  we  ought  not  to  feel  surprised  that 
it  may  arise  suddenly  or  be  suddenly  modified  in  descent.  Biol- 
ogists have  felt  it  easier  to  conceive  the  evolution  of  a  striped 


38  PROBLEMS   OF  GENETICS 

animal  like  a  zebra  from  a  self-coloured  type  like  a  horse  (or 
of  the  self-coloured  from  the  striped)  as  a  process  involving  many 
intergradational  steps;  but  so  far  as  the  pattern  is  concerned,  the 
change  may  have  been  decided  by  a  single  event,  just  as  the 
multitudinous  and  ordered  rippling  of  a  beach  may  be  created 
or  obilterated  at  one  tide. 

This  point  is  well  illustrated  by  the  tusk  of  an  Indian  elephant 
which  I  lately  found  in  a  London  sale-room.  This  tusk  is  by 
some  unknown  cause,  presumably  a  chronic  inflammation, 
thrown  up  into  thirteen  well-marked  ridges  which  closely  simulate 
a  series  of  segments  (Fig.  i).  Whatever  the  cause  the  condition 
shows  how  easily  a  normally  unsegmented  structure  may  be 
converted  into  a  series  of  repeated  parts. 

The  spread  of  segmentation  through  tissues  normally  unseg- 
mented is  very  clearly  exemplified  in  the  skates'  jaws  shown  in 
in  Fig.  2.  The  right  side  of  the  upper  figure  shows  the  normal 
arrangement  in  the  species  Rhinoptera  jussieui,  but  the  structure 
on  the  left  side  is  very  different.  The  probable  relations  of  the 
several  rows  of  teeth  to  the  normal  rows  is  indicated  by  the  let- 
tering, but  it  is  evident  that  by  the  appearance  of  new  planes 
of  division  constituting  separate  centers  of  growth,  the  series  has 
been  recast.  The  pattern  of  the  left  side  is  so  definite  that  had 
the  variation  affected  the  right  side  also,  no  systematist  would 
have  hesitated  to  give  the  specimen  a  new  specific  name.  The 
other  two  drawings  show  similar  variations  of  a  less  extensive 
kind,  the  nature  of  which  is  explained  by  the  lettering  of  the 
rows  of  teeth. 

This  power  to  divide  is  a  fundamental  attribute  of  life,  and 
of  that  power  cell-division  is  a  special  example.  In  regard  to 
almost  all  the  chief  vital  phenomena  we  can  say  with  truth  that 
science  has  made  some  progress.  If  I  mention  respiration,  meta- 
bolism, digestion,  each  of  these  words  calls  to  mind  something 
more  than  a  bare  statement  that  such  acts  are  performed  by  an 
animal  or  a  plant.  Each  stands  for  volumes  of  successful  ex- 
periment and  research,  But  the  expression  cell-division,  the 
fundamental  act  which  typifies  the  rest,  and  on  which  they  all 
depend,  remains  a  bare  name.  We  can  see  with  the  microscope 


MERISTIC  PHENOMENA 


39 


the  outward  symptoms  of  division,  but  we  have  no  surmise  as 
to  the  nature  of  the  process  by  which  the  division  is  begun  or 
accomplished.  I  know  nothing  which  to  a  man  well  f rained  in 
scientific  knowledge  and  method  brings  so  vivid  a  realisation 


E 


FIG.  2.  Jaws  of  Skates  (Rhinoptera)  showing  meristic  variation.  (For  a  detailed 
discussion  see  Materials  for  the  Sttidy  of  Variation,  p.  259.) 

of  our  ignorance  of  the  nature  of  life  as  the  mystery  of  cell- 
division.  What  is  a  living  thing?  The  best  answer  in  few  words 
that  I  know  is  one  which  my  old  teacher,  Michael  Foster,  used 
to  give  in  his  lectures  introductory  to  biology.  "A  living  thing 


40  PROBLEMS  OF  GENETICS 

is  a  vortex  of  chemical  and  molecular  change."  This  description 
gives  much,  if  not  all,  that  is  of  the  essence  of  life.  The  living 
thing  is  unlike  ordinary  matter  in  the  fact  that,  through  it,  matter 
is  always  passing.  Matter  is  essential  to  it;  but,  provided 
that  the  flow  in  and  out  is  unimpeded,  the  life-process  can  go 
on  so  far  as  we  know  indefinitely.  Yet  the  living  "vortex" 
differs  from  all  others  in  the  fact  that  it  can  divide  and  throw 
off  other  "vortices,"  through  which  again  matter  continually 
swirls. 

We  may  perhaps  take  the  parallel  a  stage  further.  A  simple 
vortex,  like  a  smoke-ring,  if  projected  in  a  suitable  way  will  twist 
and  form  two  rings.  If  each  loop  as  it  is  formed  could  grow  and 
then  twist  again  to  form  more  loops,  we  should  have  a  model 
representing  several  of  the  essential  features  of  living  things. 

It  is  this  power  of  spontaneous  division  which  most  sharply 
distinguishes  the  living  from  the  non-living.  In  the  excellent 
book  dealing  with  the  problems  of  development,  lately  published 
by  Mr.  Jenkinson  a  special  emphasis  is  very  properly  laid  on  the 
distinction  between  the  processes  of  division,  and  those  of  dif- 
ferentiation. Too  often  in  discussions  of  the  developmental 
processes  the  distinction  is  obscured.  He  regards  differentiation 
as  the  "central  difficulty."  "Growth  and  division  of  the  nucleus 
and  the  cells,"  he  tells  us,  are  side-issues.  This  view  is  quite 
defensible,  but  I  suspect  that  the  division  is  the  central  difficulty, 
and  that  if  we  could  get  a  rationale  of  what  is  happening  in 
cell-division  we  should  not  be  long  before  we  had  a  clue  to  the 
nature  of  differentiation.  It  may  be  self-deception,  but  I  do 
not  feel  it  impossible  to  form  some  hypothesis  as  to  the  mode  of 
differentiation,  but  in  no  mood  of  freest  speculation  are  we  ever 
able  to  form  a  guess  as  to  the  nature  of  the  division.  We  see 
differentiations  occurring  in  the  course  of  chemical  action,  in 
some  phenomena  of  vibration  and  so  forth:  but  where  do  we 
see  anything  like  the  spontaneous  division  of  the  living  cell? 
Excite  a  gold-leaf  electroscope,  and  the  leaves  separate,  but  we 
know  that  is  because  they  were  double  before.  In  electrolysis 
various  substances  separate  out  at  the  positive  and  negative 
poles  respectively.  Now  if  in  cell-division  the  two  daughter- 


MERISTIC   PHENOMENA  41 

cells  were  always  dissimilar — that  is  to  say,  if  differentiation 
always  occurred — we  could  conceive  some  rough  comparison 
with  such  dissociations.  But  we  know  the  dissimilarity  between 
daughter-cells  is  not  essential.  In  the  reproduction  of  unicel- 
lular organisms  and  many  other  cases,  the  products  formed  at  the 
two  poles  are,  so  far  as  we  can  tell,  identical.  Any  assumption 
to  the  contrary,  if  we  were  disposed  to  make  it,  would  involve 
us  in  difficulties  still  more  serious.  At  any  rate,  therefore,  if 
differentiation  be  really  the  central  difficulty  in  development, 
it  is  division  which  is  the  essential  problem  of  heredity. 

Sir  George  Darwin  and  Professor  Jeans  tell  us  that  "gravita- 
tional instability"  consequent  on  the  condensation  of  gases  is 
"the  primary  agent  at  work  in  the  actual  evolution  of  the  uni- 
verse," which  has  led  to  the  division  of  the  heavenly  bodies.  The 
greatest  advance  I  can  conceive  in  biology  would  be  the  dis- 
covery of  the  nature  of  the  instability  which  leads  to  the  c  ntinual 
division  of  the  cell.  When  I  look  at  a  dividing  cell  I  feel  as  an 
astronomer  might  do  if  he  beheld  the  formation  of  a  double 
star:  that  an  original  act  of  creation  is  taking  place  before  me. 
Enigmatical  as  the  phenomenon  seems,  I  am  not  without  hope 
that,  if  it  were  studied  for  its  own  sake,  dissociated  from  the 
complications  which  obscure  it  when  regarded  as  a  mere  incident 
in  development,  some  hint  as  to  the  nature  of  division  could  be 
found.  It  is  I  fear  a  problem  rather  for  the  physicist  than  for 
the  biologist.  The  sentiment  may  not  be  a  popular  one  to  utter 
before  an  assembly  of  biologists,  but  looking  at  the  truth  imper- 
sonally I  suspect  that  when  at  length  minds  of  first  rate  analytical 
power  are  attracted  to  biological  problems,  some  advance  will 
be  made  of  the  kind  which  we  are  awaiting. 

The  study  of  the  phenomena  of  bodily  symmetry  offers 
perhaps  the  most  hopeful  point  of  attack.  The  essential  fact 
in  reproduction  is  cell-division,  and  the  essential  basis  of  heredi- 
tary resemblance  is  the  symmetry  of  cell-division.  The  phenom- 
ena of  twinning  provide  a  convincing  demonstration  that  this 
is  so.  By  twinning  we  mean  the  production  of  equivalent  struc- 
tures by  division.  The  process  is  one  which  may  affect  the  whole 
body  of  an  animal  or  plant,  or  certain  of  its  parts.  The  term 


42  PROBLEMS  OF   GENETICS 

twin  as  ordinarily  used  refers  to  the  simultaneous  birth  of  two 
individuals.  Those  who  are  naturalists  know  that  such  twins 
are  of  two  kinds,  (i)  twins  that  are  not  more  alike  than  any  other 
two  members  of  the  same  family,  and  (2)  twins  that  are  so  much 
alike  that  even  intimate  friends  mistake  them.  These  latter 
twins,  except  in  imaginative  literature,  are  always  of  the  same 
sex. 

It  is  scarcely  necessary  for  me  to  repeat  the  evidence  from 
which  it  has  been  concluded  that  without  doubt  such  twins  arise 
by  division  of  the  same  fertilised  ovum.  There  is  a  perfect 
series  of  gradations  connecting  them  with  the  various  forms  of 
double  monsters  united  by  homologous  parts.  They  have  been 
shown  several  times  to  be  enclosed  in  the  same  chorion,  and  the 
proofs  of  experimental  embryology  show  that  in  several  animals 
by  the  separation  of  the  two  first  hemispheres  of  a  dividing  egg 
twins  can  be  produced.  Lastly  we  have  recently  had  the  ex- 
traordinarily interesting  demonstration  of  Loeb,  to  which  I  may 
specially  refer.  Herbst  some  years  ago  found  that  in  sea  water, 
from  which  all  lime  salts  had  been  removed,  the  segments  of  the 
living  egg  fall  apart  as  they  are  formed.  Using  this  method 
Loeb  has  shown  that  a  temporary  immersion  in  lime-free  sea 
water  may  result  in  the  production  of  90  per  cent,  of  twins. 
We  are  therefore  safe  in  regarding  the  homologous  or  "identical" 
twins  as  resulting  fro  the  divisions  of  one  fertilised  egg,  while 
the  non-identical  or  "fraternal"  twins,  as  they  are  called,  arise 
by  the  fertilisation  of  two  separate  ova.3 

In  the  resemblance  of  identical  twins  we  have  an  extreme  case 

3  These  fraternal  twins,  which  show  no  special  resemblance  to  each  other, 
are  like  the  multiple  births  of  other  animals,  and  there  is  no  disposition  for  them  to 
be  of  the  same  sex.  In  the  sheep,  for  example,  statistics  show  that  the  frequency 
of  pairs  of  twins,  male  and  female,  is  approximately  double  that  of  the  frequency 
of  pairs,  both  male  or  both  female,  as  it  should  be  if  the  sex-distribution  were  for- 
tuitous. For  instance  Bernadin  (La  Bergerie  de  Rambouillet,  1890,  p.  100)  gives 
the  following  figures  for  twin- lambs  in  Merinos:  both  male,  87;  both  female,  83; 
sexes  mixed,  187.  The  p-banded  Armadillo  (Dasypus  novemcinctus) ,  in  which 
the  young  born  in  one  litter  are  said  to  be  always  of  one  sex,  is  the  only  known 
exception  in  Vertebrates,  and  is  presumably  a  genuine  case  of  normal  polyembryony 
(see  especially,  Rosner,  Bull.  Ac.  Soc.  Cracovie,  1901,  p.  443,  and  Newman  and 
Patterson,  Biol.  Bull.,  XVII,  1909,  p.  181,  and  an  important  paper  lately  published 
by  H.  H.  Newman  and  J.  T.  Patterson,  Jour.  Morph.,  1911,  XXII,  p.  855. 


MERISTIC   PHENOMENA  43 

of  hereditary  likeness4  and  a  proof,  if  any  were  needed,  that  the 
cause  of  individual  variation  is  to  be  sought  in  the  differentiation 
of  germ-cells.  The  resemblance  of  identical  twins  depends  on 
two  circumstances,  First,  since  only  two  germ-cells  take  part 
in  their  production,  difference  between  the  germ  cells  of  the 
same  individual  cannot  affect  them.  Secondly  the  division  of 
the  fertilised  ovum,  the  process  by  which  they  became  two  in- 
stead of  one,  must  have  been  a  symmetrical  division.  The 
structure  of  twins  raises  however  one  extremely  significant 
difficulty,  which  as  yet  we  cannot  in  any  way  explain.  The 
resemblance  between  twins  is  a  phenomenon  of  symmetry, 
like  the  resemblance  between  the  two  sides  of  a  bilaterally  sym- 
metrical body.  Not  only  is  the  general  resemblance  readily 
so  interpreted,  but  we  know  also  that  in  double  monsters,  namely 
unseparated  twins,  various  anatomical  abnormalities  shown  by 
the  one  half-body  are  frequently  shown  by  the  other  half- 
also. 5  The  two  belong  to  one  system  of  symmetry  How  then 
does  it  happen  that  the  body  of  one  of  a  pair  of  twins  does  not 
show  a  transposition  of  viscera?  We  know  that  the  relation  of 
right  and  left  implies  that  the  one  should  be  the  mirror-image  of 
the  other.  Such  a  relation  of  images  may  be  maintained  even 
in  minute  details.  For  example  if  the  same  pattern  of  finger- 
print is  given  by  the  fingers  of  the  two  hands,  one  is  the  reverse 
of  the  other.  In  double  monsters,  namely  unseparated  twins, 
there  is  evidence  that  an  inversion  of  viscera  does  occur  with 
some  frequency.  Evidence  from  such  cases  is  not  so  clear  and 
simple  as  might  be  expected,  because  as  a  matter  of  fact,  the 
heart  and  stomach,  upon  which  the  asymmetry  of  the  viscera 
chiefly  depend,  are  usually  common  to  the  two  bodies.  Du- 
plicity generally  affects  either  the  anterior  end  alone,  or  the  pos- 
terior end  alone.  The  division  is  generally  from  the  heart  forwards , 
giving  two  heads  and  two  pairs  of  anterior  limbs  on  a  common 
trunk,  or  from  the  heart  backwards,  giving  two  pairs  of  posterior 
limbs  with  the  anterior  body  common.  In  either  case,  though 

*  A  good  collection  of  evidence  as  to  disease  in  homologous  twins  was  lately 
published  by  E.  A.  Cockayne,  Brit.  Jour.  Child.  Diseases,  Nov.,  1911. 
6  Cp.  Windle,  B.  C.  A.,  Jour.  Anat.  Phys.,  XXVI,  p.  295. 


44  PROBLEMS   OF  GENETICS 

the  bodies  may  be  grouped  in  a  common  system  of  symmetry, 
neither  can  be  proved  to  show  definite  reversal  of  the  parts.  To 
see  that  reversal  recourse  must  be  had  to  more  extreme  duplica- 
tions, such  as  the  famous  Siamese  Twins.  They,  as  a  matter  of 
fact,  were  an  excellent  instance  of  the  proposition  that  twins 
are  related  as  mirror-images,  for  both  of  them  had  eleven  pairs 
of  ribs  instead  of  the  normal  twelve,  and  one  of  them  had  a 
partial  reversal  of  viscera.6  (Kiichenmeister,  Verlagerung,  etc., 
p.  204.) 

If  anyone  could  show  how  it  is  that  neither  of  a  pair  of 
twins  has  transposition  of  viscera  the  whole  mystery  of  division 
would,  I  expect,  be  greatly  illuminated.7  At  present  we  have 
simply  to  accept  the  fact  that  twins,  by  virtue  of  their  detach- 
ment from  each  other,  have  the  power  of  resuming  the  polarity 
which  is  proper  to  any  normal  individual.  It  was  nevertheless 
with  great  interest  that  I  read  Wilder's  recent  observation 8  that 
occasionally  in  identical  twins  the  finger-print  of  one  or  both 
the  index-fingers  may  be  reversed,  showing  that  there  is  after 
all  some  truth  in  the  notion  that  reversal  should  occur  in  them. 

There  is  another  phenomenon  by  twinning  which,  if  we  could 
understand  it,  might  help.  I  refer  to  the  free-martin,  the  subject 
of  one  of  John  Hunter's  masterpieces  of  anatomical  description. 
In  horned  cattle  twin  births  are  rare,  and  when  twins  of  opposite 
sexes  are  born,  the  male  is  perfect  and  normal,  but  the  repro- 

6  Mr.  E.  Nettleship  tells  me  that  in  the  course  of  collecting  pedigrees  of  families 
containing  colour-blind  members  he  has  discovered  two  cases  (shortly  to  be  pub- 
lished) of  pairs  of  twins,  which  on  account  of  their  very  close  resemblances  must 
be  deemed  homologous,  one  of  each  pair  being  colour-blind  and  the  other  normal. 
Such  a  distinction  between  closely  similar  twins  is  most  curious  and  unexpected. 

7  Another  paradoxical  phenomenon  of  the  same  nature  occurs  in  the  Narwhal 
The  males  normally  have  the  left  tusk  alone  developed,  the  corresponding  right 
tusk  remaining  as  an  undeveloped  rudiment  in  its  socket.      The  left  tusk  is  a 
left-handed  screw.     Occasionally  the  right  tusk  is  also  developed  and  grows  to 
the  same  length  as  that  of  the  left  side,  but  in  such  specimens  the  right  tusk  is 
also  a  left-hand  screw  like  the  tusk  of  the  other  side,  instead  of  being  reversed 
as  we  should  certainly  have  expected.     It  need  scarcely  be  remarked  that  in  the 
case  of  the  horns  of  antelopes,  and  in  other  examples  of  spiral  organs  arranged  in 
pairs,  that  of  one  side  of  the  body  is  the  mirror  image  of  that  on  the  other  side. 
The  Narwhal's  tusks  in  being  both  twisted  in  the  same  direction  are  thus  highly 
anomalous,  and  are  comparable  with  pairs  of  twins. 

8  Wilder,  H.  H.,  Amer.  Jour.  Anat.,  1904,  III,  p.  452. 


MERISTIC  PHENOMENA  45 

ductive  organs  of  the  female  are  deformed  and  sterile,  being 
known  as  a  free-martin.  The  same  thing  occasionally  happens 
in  sheep,  suggesting  that  in  sheep  also  twins  may  be  formed  by 
the  division  of  one  ovum;  for  it  is  impossible  to  suppose  that 
mere  development  in  juxtaposition  can  produce  a  change  of  this 
character.  I  mention  the  free-martin  because  it  raises  a  question 
of  absorbing  interest.  It  is  conceivable  that  we  should  interpret 
it  by  reference  to  the  phenomenon  of  gynandromorphism,  seen 
occasionally  in  insects,  and  also  in  birds  as  a  great  rarity.  In 
the  gynandromorph  one  side  of  the  body  is  male,  the  other  female. 
A  bullfinch  for  instance  has  been  described  with  a  sharp  line  of 
division  down  the  breast  between  the  red  feathers  of  the  cock 
on  one  side  and  the  brown  feathers  of  the  hen  on  the  other. 
(Poll,H.,  SB.  Ges.  Nat.  Fr.t  Berlin,  1909,  p.  338.)  In  such  cases 
neither  side  is  sexually  perfect.  If  the  halves  of  such  a  gynan- 
dromorph came  apart,  perhaps  one  would  be  a  free-martin. 

The  behaviour  of  homologous  twinning  in  heredity  has  been 
little  studied.  It  does  not  exist  as  a  normal  feature  in  any  animal 
which  is  amenable  to  experiment,  and  we  cannot  positively 
assert  that  a  comparable  phenomenon  exists  in  plants;  for  in 
them — the  Orange,  for  example — polyembryony  may  evidently 
be  produced  by  a  parthenogenetic  development  of  nucellar  tissue. 
It  is  possible  that  in  Man  twinning  is  due  to  a  peculiarity  of  the 
mother,  not  of  the  father.  It  may  and  not  rarely  does  descend 
from  mother  to  daughter,  but  whether  it  can  be  passed  on 
through  a  male  generation  to  a  daughter  again,  there  is  not 
sufficient  evidence  to  show.  The  facts  as  far  as  they  go  are 
consistent  with  the  inference  which  may  be  drawn  from  Loeb's 
experiment,  that  the  twinning  of  a  fertilized  ovum  may  be  de- 
termined not  by  the  germ-cells  which  united  to  form  it,  but  by 
the  environment  in  which  it  begins  to  develop.  The  opinion  that 
twinning  may  descend  through  the  male  directly  has  been  lately 
expressed  by  Dr.  J.  Oliver  in  the  Eugenics  Review  (1912),  on  the 
evidence  of  cases  in  which  twins  had  occurred  among  the  rela- 
tions of  fathers  of  twins,  but  I  do  not  know  of  any  comprehen- 
sive collection  of  evidence  bearing  on  the  subject. 

Besides  twinning  of  the  whole  body  a  comparable  duplicity 


46  PROBLEMS  OF  GENETICS 

of  various  parts  of  the  same  body  may  occur.  Such  divisions 
affect  especially  those  organs  which  have  an  axis  of  bilateral 
symmetry,  such  as  the  thumb,  a  cotyledon,  a  median  petal, 
the  frond  of  a  fern  or  the  anal  fin  of  a  fish.  From  the  little 
yet  known  it  is  clear  that  the  genetic  analysis  of  these  conditions 
must  be  very  difficult,  but  evidence  of  any  kind  regarding  them 
will  be  valuable.  We  want  especially  to  know  whether  these 
divisions  are  due  to  the  addition  of  some  factor  or  power  which 
enables  the  part  to  divide,  or  whether  the  division  results  from 
the  absence  of  something  which  in  the  normal  body  prevents 
the  part  from  dividing.  Breeding  experiments,  so  far  as  they 
go,  suggest  that  the  less  divided  state  is  usually  dominant  to 
the  more  divided.9  The  two-celled  Tomato  fruit  is  dominant  to 
the  many-celled  type.  The  Manx  Cat's  tail,  with  its  suppression 
of  caudal  segmentation  is  a  partial  dominant  over  the  normal 
tail.  The  tail  of  the  Fowl  in  what  is  called  the  "Rumpless" 
condition  is  at  least  superficially  comparable  with  that  of  the 
Manx  Cat,  and  though  the  evidence  is  not  wholly  consistent, 
Davenport  obtained  facts  indicating  that  this  suppressed  con- 
dition of  the  caudal  vertebrae  is  an  imperfect  dominant.10 

Some  evidence  may  also  be  derived  from  other  examples  of 
differences  which  at  first  sight  appear  to  be  substantive  though 
they  are  more  probably  meristic  in  ultimate  nature.  The 
distinction  between  the  normal  and  the  "Angora"  hair  of  the 
Rabbit  is  a  case  in  point.  We  can  scarcely  doubt  that  one  of 
the  essential  differences  between  these  two  types  is  that  in  the 
Angora  coat  the  hair-follicles  are  more  finely  divided  than  they 
are  in  the  normal  coat,  and  we  know  that  the  normal,  or  less- 
divided  condition,  is  dominant  to  the  Angora,  or  more  finely 
divided. 

In  the  case  of  the  solid-hoofed  or  "mule-footed"  swine,  the 

9  Polydactylism  which  is  often  a  dominant  and  the  web-foot  of  Pigeons  which 
is  recessive  should  be  remembered  as  possible  exceptions  (see  p.  49). 

10  Davenport  inclined  at  first  to  regard  rumplessness  as  a  recessive,  but  in  his 
latest  publication  on  the  subject  he  definitely  concludes  that  it  is  an  imperfect 
dominant.     This  conclusion  accords  well  with  evidence  quoted  by  Darwin  (An. 
and  Pits.,  II,  ed.  2,  p.  4)  that  rumpless  fowls  may  throw  tailed  offspring.     (Amer. 

..  1910.  XLIV,  p.  134-) 


MERISTIC   PHENOMENA 


47 


evidence  shows,  as  Spillman  has  lately  pointed  out,u  that  the 
condition  behaves  as  a  dominant.  The  essential  feature  of 
this  abnormality  is  that  the  digits  III  and  IV  are  partially 
united.  The  union  is  greatest  peripherally.  Sometimes  the 


n 


IV 


FIG.  3.  /,  //,  ///,  various  degrees  of  syndactyly  affecting  the  medius  and 
annularis  in  the  hand;  IV,  syndactyly  affecting  the  index  and  medius  in  the  foot. 
(After  Annandale.) 

third  phalanges  only  are  joined  to  form  one  bone,  but  the  second 
and  even  the  first  phalanges  may  also  be  compounded  together. 
Here  the  variation  is  obviously  meristic  and  consists  in  a  failure 

"Spillman,  W.  J.,  Amer.  Breeders  Mag.,  1910,  I,  p.  178. 


48  PROBLEMS  OF  GENETICS 

to  divide,  the  normal  separation  of  the  median  digits  of  the  foot 
being  suppressed. 

Webbing  between  the  digits,  in  at  least  some  of  its  mani- 
festations, is  a  variation  of  similar  nature.  The  family  recorded 
by  Newsholme12  very  clearly  shows  the  dominance  of  this  con- 
dition. The  case  is  morphologically  of  great  interest  and  must 
undoubtedly  have  a  bearing  on  the  problems  of  the  mechanics 
of  Division.  In  discussing  the  phenomena  of  syndactylism 
I  pointed  out  some  years  ago  that  the  digits  most  frequently 
united  in  the  human  hand  are  III  and  IV,  while  in  the  foot, 


a 


_     m+in 

'v 

FIG.  4.  Case  of  complete  syndactyly  in  the  foot.  77  and  777,  digit  apparently 
representing  the  index  and  medius.  c2  +  cs,  bone  apparently  representing  the 
middle  and  external  cuneiform;  cb,  cuboid;  cl,  internal  cuneiform.  (After  Gruber.) 

union  most  frequently  takes  place  between  II  and  III.13  In 
Newsholme's  family  the  union  was  always  between  II  and  III 
of  the  foot,  except  in  the  case  of  one  male  who  had  the  digits 
III  and  IV  of  the  right  hand  alone  webbed  together.  There 
can  be  little  doubt  that  the  geometrical  system  on  which  the 
foot  is  planned  has  an  axis  of  symmetry  passing  between  the 
digits  II  and  III,  while  the  corresponding  axis  in  the  hand  passes 
between  III  and  IV.  Union  between  such  digits  may  therefore 
be  regarded  as  comparable  with  any  non-division  or  "  coalescence  " 
of  lateral  structures  in  a  middle  line,  and  when  as  in  these  ex- 

12  Newsholme,  Lancet,  December  10,  1910,  p.  1690. 
18  Materials  for  the  Study  of  Variation,  1894,  p.  358. 


MERISTIC   PHENOMENA  49 

amples  such  a  condition  is  shown  to  be  a  dominant  we  cannot 
avoid  the  inference  that  some  concrete  factor  has  the  power  of 
suppressing  or  inhibiting  this  division.  Figs.  3  and  4  illus- 
trate degrees  of  union  between  digits  in  the  human  hand  and 
foot. 

It  is  not  in  question  that  various  other  forms  of  irregular 
webbing  and  coalescence  of  digits  exist,  and  respecting  the  genetic 
behaviour  of  these  practically  nothing  is  as  yet  known,  Such 
a  case  is  described  by  Walker,14  in  which  the  first  and  second 
metacarpals  of  both  feet  were  fused  in  mother  and  daughter, 
and  several  more  are  found  in  literature.  Contrasted  with  these 
phenomena  we  have  the  curious  fact  that  in  the  Pigeon,  Staples- 
Browne  found  webbing  of  the  toes  a  recessive  character.  The 
question  thus  arises  whether  this  webbing  is  of  the  same  nature 
as  that  shown  to  be  a  dominant  in  Man,  and  indeed  whether  the 
phenomenon  in  pigeons  is  really  meristic  at  all.  There  is  some 
difference  perceptible  between  the  two  conditions;  for  in  Man 
there  is  not  so  much  a  development  of  a  special  web-like  skin 
uniting  the  digits  as  a  w^ant  of  proper  division  between  the  digits 
themselves,  and  in  extreme  cases  two  digits  may  be  represented 
by  a  single  one.  In  the  Pigeon  I  am  not  aware  that  a  real 
union  of  this  kind  has  ever  been  observed,  and  though  the  web-like 
skin  may  extend  the  whole  length  of  the  digits  and  be  so  narrow 
as  to  prevent  the  spread  of  the  toes,  it  may,  I  think,  be  main- 
tained that  the  unity  of  the  digits  is  unimpaired.  For  the 
present  the  nature  of  this  variation  in  the  pigeon's  feet  must  be 
regarded  as  doubtful,  and  we  should  note  that  if  it  is  actually 
an  example  of  a  more  perfect  division  being  dominant  to  a  less 
perfect  division,  the  case  is  a  marked  exception  to  the  general 
rule  that  non-division  is  dominant  to  division. 

Reference  must  also  be  made  to  the  phenomenon  of  fasciation 
in  the  stems  of  plants.  As  Mendel  showed  in  the  case  of  Pisum 
this  condition  is  often  a  recessive.  The  appearances  suggest 
that  the  difference  between  a  normal  and  a  fasciated  plant 
consists  in  the  inability  of  the  fasciated  plant  to  separate  its 
lateral  branches.  The  nature  of  the  condition  is  however  very 

14  Walker,  G.,  Johns  Hopkins  Hospital  Bulletin,  XII,  1901,  p.  129. 

5 


So  PROBLEMS  OF  GENETICS 

obscure  and  it  is  equally  likely  that  some  multiplication  of  the 
growing  point  is  the  essential  phenomenon.16 

Stockard's  interesting  experiments16  illustrate  this  question. 
He  showed  that  by  treating  the  embryos  of  a  fish  (Fundulus 
heteroclitus)  with  a  dilute  solution  of  magnesium  salts,  various 
cyclopian  monstrosities  were  frequently  produced.  These  have 
been  called  cases  of  fusion  of  the  optic  vesicles.  I  would  prefer 
to  regard  them  as  cases  of  a  division  suppressed  or  restricted  by 
the  control  of  the  environment.  Conversely,  the  splendid  dis- 
covery of  Loeb,  that  an  unfertilised  egg  will  divide  and  develop 
parthenogenetically  without  fertilisation,  as  a  consequence  of 
exposure  to  various  media,  may  be  interpreted  as  suggesting  that 
the  action  of  those  media  releases  the  strains  already  present 
in  the  ovum,  though  I  admit  that  an  interpretation  based  on  the 
converse  hypothesis,  that  the  medium  acts  as  a  stimulus,  is  as 
yet  by  no  means  excluded. 

In  these  cases  we  come  nearest  to  the  direct  causation  or 
the  direct  inhibition  of  a  division,  but  the  meaning  of  the 
evidence  is  still  ambiguous.  I  incline  to  compare  Loeb's  par- 
thenogenesis with  the  development  (and  of  course  accompany- 
ing cell-division)  of  dormant  buds  on  stems  which  have  been 
cut  back. 

It  is  interesting  to  note  that  sometimes  as  an  abnormality, 
the  faculty  of  division  gets  out  of  hand  and  runs  a  course  ap- 
parently uncontrolled.  A  remarkable  instance  of  this  condition 
is  seen  in  Begonia  "  phyllomaniaca"  which  breaks  out  into  buds 
at  any  point  on  the  stem,  petioles,  or  leaves,  each  bud  having, 
like  other  buds,  the  power  of  becoming  a  new  plant  if  removed. 
We  would  give  much  to  know  the  genetic  properties  of  B.  phyl- 
lomaniaca,  and  in  conjunction  with  Mr.  W.  O.  Backhouse  I  have 
for  some  time  been  experimenting  with  this  plant.  It  proved 
totally  sterile.  Its  own  anthers  produce  no  pollen,  and  all  at- 
tempts to  fertilise  it  with  other  species  failed  though  the  pollen 
of  a  great  number  of  forms  was  tried. 

Recently  however  we  have  succeeded  in  making  plants  which 

15  Cp.  R.  H.  Compton,  New  Phytologist,  1911,  p.  249. 

16  Arch,  f,  Entwickelungsmech.,  1907,  XXIII,  p.  249. 


MERISTIC  PHENOMENA  51 

are  in  every  respect  Begonia  phyllomaniaca,  so  far  as  the  char- 
acters of  stems  and  leaves  are  concerned.  These  plants,  of 
which  we  have  sixteen,  were  made  by  fertilising  B.  heracleifolia 
with  B.  polyantha.  They  are  all  beginning  to  break  out  in 
"phyllomania."  As  yet  they  have  not  flowered,  but  as  they 
agree  in  all  details  with  phyllomaniaca  there  can  be  little  doubt 
that  the  original  plant  bearing  that  name  was  a  hybrid  similarly 
produced.  The  production  of  "phyllomania"  on  a  hybrid 
Begonia  has  also  been  previously  recorded  by  Duchartre.17 
In  this  case  the  cross  was  made  between  B.  incarnata  and  lucida. 
The  synonymy  of  the  last  species  is  unfortunately  obscure,  and 
I  have  not  succeeded  in  repeating  the  experiment. 


FIG.  5.     Piece  of  petiole  of  Begonia  phyllomaniaca.    The  proximal  end  is  to  the 

right  of  the  figure. 

From  these  facts  it  seems  practically  certain  that  the  condition 
is  one  which  is  due  to  the  meeting  of  complementary  factors. 
At  first  sight  we  may  incline  to  think  that  the  phyllomania  is  in 
some  way  due  to  the  sterility.  This  however  cannot  be  seriously 
maintained;  for  not  only  is  sterility  in  plants  not  usually  associ- 
ated with  such  manifestations,  but  we  know  a  Begonia  called 
"Wilhelma"  which  is  exactly  phyllomaniaca  and  equally  sterile, 
though  it  has  no  trace  of  phyllomania.  This  plant  arose  in  the 
nurseries  of  MM.  P.  Bruant  of  Poitiers,  and  has  generally  been 
described  as  a  seedling  of  phyllomaniaca,  but  from  the  total 
sterility  of  that  form  this  account  of  its  origin  must  be  set  aside. 

The  phenomenon  in  this  case  can  hardly  be  regarded  as 

17  Bull.  Soc.  Bot.  de  France,  xxxiv,  1887,  p.  182. 


PROBLEMS  OF  GENETICS 


due  to  the  excitation  of  dormant  buds,  for  it  is  apparent  on 
examination  that  the  new  growths  are  not  placed  in  any  fixed 
geometrical  relation  to  the  original  plant.  They  arise  on  the 


tic 


a» 


c* 
s» 

c* 


d* 


d*. 


'<t2 


FIG.  6.  Two  right  hind  feet  of  polydactyle  cats.  //  shows  the  lowest  de- 
velopment of  the  condition  yet  recorded.  The  digit,  di,  which  stands  as  hallux  is 
fully  formed  and  has  three  phalanges.  Both  it  and  the  digit  marked  fa  are  formed 
as  left  digits.  In  the  normal  hind  foot  of  the  cat  the  hallux  is  represented  by  a 
rudiment  only. 

I  shows  a  further  development  of  the  condition.  In  this  foot  there  are  six 
digits,  di  has  two  phalanges,  but  both  it  and  d2  and  d*  are  shaped  as  left  digits. 
Thus  d3,  which  in  the  normal  foot  would  be  shaped  as  a  right  digit,  is  transformed 
so  as  to  look  like  a  left  digit. 

petiole,  for  example,  as  small  green  outgrowths  each  of  which 
gradually  becomes  a  tiny  leaf.  The  attitude  of  these  leaves  is 
quite  indeterminate,  and  they  may  point  in  any  direction, 


MERISTIC  PHENOMENA  53 

some  having  their  apices  turned  peripherally,  some  centrally,  and 
others  in  various  oblique  or  transverse  positions  (Fig.  5).  These 
little  leaves  are  thus  comparable  with  seedlings,  in  that  their 
polarity  is  not  related  to,  or  consequent  upon  that  of  the  parent 
plant.  They  have  in  fact  that  "individuality,"  which  we  asso- 
ciate with  germinal  reproduction. 

There  are  many  curious  phenomena  seen  in  the  behaviour  of 
parts  normally  repeated  in  bilateral  symmetry  which  may  some 
day  guide  us  towards  an  understanding  of  the  mechanics  of 
division.  A  part  like  a  hand,  which  needs  the  other  hand  to 
complete  its  symmetry,  cannot  twin  by  mere  division,  yet  by 
proliferation  and  special  modifications  on  the  radial  side  of  the 
same  limb,  even  a  hand  may  be  twinned.  In  the  well  known  poly- 
dactyle  cats  a  change  of  this  kind  is  very  common  and  indeed 
almost  the  rule.  When  extra  digits  appear  at  the  inner  (tibial) 
side  of  the  limb,  they  are  shaped  as  digits  of  the  other  side,  and 
even  the  normal  digit  II  (index)  is  usually  converted  into  the 
mirror-image  of  its  normal  self.  The  limb  then  develops  a 
new  symmetry  in  itself.  Nevertheless  it  is  not  easy  to  interpret 
these  facts  as  meaning  that  there  has  been  some  interruption 
in  the  control  which  one  side  of  the  body  exercises  over  the 
other.  The  heredity  of  polydactylism  is  complex  but  there  is 
little  doubt  that  the  condition  familiar  in  the  Cat  is  a  dominant. 
In  some  human  cases  also  the  descent  is  that  of  a  dominant,  but 
irregularities  are  so  frequent  that  no  general  rule  can  yet  be 
perceived.  The  dominance  of  such  a  condition  is  an  exception 
to  the  principle  that  the  less-divided  is  usually  dominant  to  the 
more-divided,  a  fact  which  probably  should  be  interpreted  as 
meaning  that  divisions  are  of  more  than  one  kind. 

Among  ordinary  somatic  divisions,  whether  of  organs,  cells, 
or  patterns  of  differentiation,  the  control  of  symmetry  is  usually 
manifested.  There  is  however  one  class  of  somatic  differenti- 
ations which  are  exceptionally  interesting  from  the  fact  that  they 
may  show  a  complete  independence  of  such  geometrical  control. 
The  most  familiar  examples  of  these  geometrically  uncontrolled 
Variations  are  to  be  seen  in  bud-sports.  The  normal  differ- 
entiation of  the  organs  of  a  plant  is  arranged  on  a  definite  geo- 


54  PROBLEMS  OF  GENETICS 

metrical  system,  which  to  those  who  have  never  given  special 
attention  to  such  things  before,  will  often  seem  surprisingly 
precise.  The  arrangement  of  the  leaves  on  uninjured,  free- 
growing  shoots  can  generally  be  seen  to  follow  a  very  definite 
order,  just  as  do  the  flowers  or  the  parts  of  the  flowers.  If 
however  bud  sports  occur,  then  though  the  parts  included  in 
the  sports  show  all  the  geometrical  peculiarities  proper  to  the 
sport- variety,  yet  the  sporting-buds  themselves  are  not  related 
to  each  other  according  to  any  geometrical  plan. 

A  very  familiar  illustration  is  provided  by  the  distribution 
of  colour  in  those  Carnations  that  are  not  self-coloured.  The 
pigment  may,  as  in  Picotees,  be  distributed  peripherally  with 
great  regularity  to  the  edges  of  the  petals;  or,  as  in  Bizarres  and 
Flakes,  it  may  be  scattered  in  radial  sectors  which  show  no 
geometrical  regularity.  Now  in  this  case  the  pigments  are  the 
same  in  both  types  of  flower,  and  the  chemical  factors  concerned 
in  their  production  must  surely  be  the  same.  The  difference 
must  lie  in  the  mechanical  processes  of  distribution  of  the  pig- 
ment. In  the  Picotee  we  see  the  orderly  differentiation  which  we 
associate  with  normality;  in  the  Bizarre  we  see  the  disorderly 
differentiation  characteristic  of  bud-sports.  The  distribution  of 
colour  in  this  case  lies  outside  the  scheme  of  symmetry  of  the 
plant. 

Such  a  distribution  is  characteristic  of  bud-sports,  and  of 
certain  other  differentiations  in  both  plants  and  animals,  which 
I  cannot  on  this  occasion  discuss.  Now  reflexion  will  show  that 
these  facts  have  an  intimate  bearing  on  the  mechanical  problems 
of  heredity.  For  first  in  the  bud-sports  we  are  witnessing  the 
distribution  of  factors  which  distinguish  genetic  varieties.  We 
do  not  know  the  physical  nature  of  those  factors,  but  if  we  must 
give  them  a  name,  I  suppose  we  should  call  them  "ferments" 
exactly  as  Boyle  did  in  1666.  He  is  discussing  how  it  comes  about 
that  a  bud,  budded  on  a  stock,  becomes  a  branch  bearing  the 
fruit  of  its  special  kind.  He  notes  that  though  the  bud  inserted 
be  "not  so  big  oftentimes  as  a  Pea,"  yet  "whether  by  the  help 
of  some  peculiar  kind  of  Strainer  or  by  the  Operation  of  some 
powerful  Ferment  lodged  in  it,  or  by  both  these,  or  some  other 


MERISTIC  PHENOMENA  55 

cause,"  the  sap  is  "so  far  changed  as  to  constitute  a  Fruit  quite 
otherwise  qualify 'd."  18  We  can  add  nothing  to  his  speculation, 
and  we  believe  still  that  by  a  differential  distribution  of  "fer- 
ments" the  sports  are  produced.  All  the  factors  are  together 
present  in  the  normal  parts;  some  are  left  out  in  the  sport. 
In  an  analogous  case  however,  that  of  a  variegated  Pelargonium 
which  has  green  and  also  albino  shoots,  Baur  proved  that  the 
shoots  pure  in  colour  are  also  pure  in  their  posterity.  There 
can  be  no  doubt  that  the  sports  of  Carnations,  Azaleas,  Chrysan- 
themums, etc.,  would  behave  in  the  same  way. 

The  well-known  Azaleas  Perle  de  Ledeburg,  President 
Kerchove,  and  Vervaeana  are  familiar  illustrations.  Perle  de 
Ledeburg  is  predominantly  white,  but  it  has  red  streaks  in  some 
of  its  flowers.  It  not  very  rarely  gives  off  a  self-red  sport.  This 
is  evidently  due  to  the  development  of  a  bud  in  a  red-bearing 
area  of  the  stem.  The  red  in  this  plant  is  not  under  "geometrical 
control."  Many  plants  have  white  flowers  with  no  markings, 
but  if  the  red  markings  are  geometrically  ordered  differentiations, 
no  self-coloured  sports  are  formed.  The  case  of  Vervaeana  is  a 
good  illustration  of  this  proposition.  It  has  white  flowers  with 
red  markings  arranged  in  an  orderly  manner  on  the  lower  parts 
of  the  petals,  especially  on  the  dorsal  petals.  This  is  one  of  the 
Azaleas  most  liable  to  have  red  sports,  and  at  first  sight  it  might 
seem  that  the  sport  represented  the  red  of  the  central  marks. 
Examination  however  of  a  good  many  flowers  shows  that  irregular 
red  streaks  like  those  of  Perle  de  Ledeburg  occur,  about  as  com- 
monly as  in  that  variety.  Vervaeana  in  fact  is  Perle  de  Ledeburg 
with  definite  red  markings  added,  and  its  red  sports  obviously  are 
those  branches  the  germs  of  which  came  in  a  patch  of  the  stem 
bearing  these  red  elements.  That  this  is  the  true  account  is 
rendered  quite  obvious  by  the  fact  that  the  red  of  the  sport  is  a 
colour  somewhat  different  from  that  of  the  definite  marks,  and 
that  these  marks  are  still  present  on  the  red  ground  of  the  sporting 
flowers. 

It  will  be  understood  that  these  remarks  apply  to  those  cases 
in  which  the  production  of  sports  is  habitual  or  frequent,  and 

18  R.  Boyle,  The  Origine  of  Formes  and  Qualities,  Oxford,  1666. 


56  PROBLEMS  OF  GENETICS 

I  imagine  in  all  such  examples  it  will  be  found  that  there  are 
indications  of  irregularity  in  the  distribution  of  the  differ- 
entiations such  as  to  justify  the  view  that  they  are  not  under  that 
geometrical  control  which  governs  the  normal  differentiation 
of  the  parts.  The  question  next  arises  whether  these  consider- 
ations apply  also  to  the  production  of  a  bud-sport  as  a  rare 
exception,  but  by  the  nature  of  the  case  it  is  not  possible  to 
say  positively  whether  the  appearance  of  an  exceptional  sport 
is  due  to  the  unsuspected  presence  of  a  pre-existing  fragment  of 
material  having  a  special  constitution,  or  to  the  origin,  de  novo, 
of  such  a  material.  For  instance  one  of  the  garden  forms  of 
Pelargonium  known  as  altum  is  liable  perhaps  once  in  some 
hundreds  of  flowers  to  have  one  or  two  magenta  petals.  The 
normal  colour  is  a  brilliant  red ;  and  as  we  may  be  fairly  sure  that 
this  red  is  recessive  to  magenta  the  interpretation  would  be 
quite  different  according  as  the  appearance  of  the  magenta  is 
regarded  as  due  to  the  presence  of  small  areas  endowed  with 
magentaness,  or  to  the  spontaneous  generation  of  the  factor 
for  that  pigment.  Either  interpretation  is  possible  on  the  facts, 
but  the  view  that  the  whole  plant  has  in  it  scarce  mosaic  particles 
of  magenta  seems  on  the  whole  more  consistent  with  present 
knowledge. 

In  Pelargonium  altum  the  enzyme  causing  the  magenta  colours 
must  be  distributed  in  very  small  areas,  but  a  case  in  which  the 
magenta  is  similarly  arranged  in  a  much  coarser  patchwork 
may  be  seen  in  the  Pelargonium  "  Don  Juan,"  which  often  bears 
whole  trusses  or  branches  of  red  flowers  upon  plants  having  the 
normal  dominant  magenta  trusses.  In  most  cases  there  is  little 
doubt  that  though  the  magenta  flowered  parts  can  "  sport"  to 
red,  the  red  parts  could  not  produce  the  magenta  flowers. 

The  asymmetrical,  or  to  speak  more  precisely,  the  disorderly, 
mingling  of  the  colours  in  the  somatic  parts  is  thus  an  indication 
of  a  similarly  disorderly  mixing  of  the  factors  for  those  colours 
in  the  germ-tissues,  so  that  some  of  the  gametes  bear  enough  of 
the  colour-factors  to  make  a  self-coloured  plant,  while  others 
bear  so  little  that  the  plant  to  which  they  give  rise  is  a  patch- 
work. If  this  view  is  correct  we  may  extend  it  so  far  as  to  con- 


MERISTIC  PHENOMENA  57 

sider  whether  the  fineness  or  coarseness  of  the  mixture  visible 
in  the  flowers  or  leaves  may  not  give  an  indication  of  the  degree 
to  which  the  factors  are  subdivided  among  the  germ-cells.  We 
know  very  little  about  the  genetic  properties  of  striped  varieties. 
In  both  Antirrhinum  and  Mirabilis  it  has  been  found  that  the 
striped  may  occasionally  and  irregularly  throw  self-coloured 
plants,  and  therefore  the  striping  cannot  be  regarded  simply  as 
a  recessive  character.  On  the  other  hand  in  Primula  Sinensis 
there  are  well-known  flaked  varieties  which  ordinarily  at  least 
breed  true.  Whether  these  ever  throw  selfs  I  do  not  know, 
but  if  they  do  it  must  be  quite  exceptionally.  The  power  of 
these  flaked  plants  to  breed  true  is,  I  suspect,  connected  with 
the  fact  that  in  their  flowers  the  coloured  and  white  parts  are 
intimately  mixed,  this  intimate  mixture  thus  being  an  indication 
of  a  similarly  intim  te  mixture  in  the  germ-cells.  It  would  be 
important  to  ascertain  whether  self-fertilised  seed  from  the  oc- 
casional flowers  in  which  the  colour  has  run  together  to  join  a 
large  patch  gives  more  self-coloured  plants  than  the  intimately 
flaked  flowers  do. 

The  next  fact  may  eventually  prove  of  great  importance. 
We  have  seen  that  in  bud-sports  the  differentiation  is  of  the  same 
nature  as  that  between  pure  types,  and  also  that  in  the  sporting 
plant  this  differentiation  is  distributed  without  any  reference 
to  the  plant's  axis,  or  any  other  consideration  of  symmetry. 
Now  among  the  germ-cells  of  a  Mendelian  hybrid  exactly  such 
characters  are  being  distributed  allelomorphically,  and  there 
again  we  have  strong  evidence  for  believing  that  the  distribution 
obeys  no  pattern.  For  example,  we  can  in  the  case  of  seeds  still 
in  situ  perceive  how  the  characters  were  distributed  among  the 
germ-cells,  and  there  is  certainly  no  obvious  pattern  connecting 
them,  nor  can  we  suppose  that  there  is  an  actual  pattern  obscured. 

Of  this  one  illustration  is  especially  curious.  Individual 
plants  of  the  same  species  are,  as  regards  the  decussations  of 
their  leaves  and  in  other  respects,  either  rights  or  lefts.  The  fact 
is  not  emphasized  in  modern  botany  and  is  in  some  danger  of 
being  forgotten.  When,  as  in  the  flowers  of  Arum,  some  Gladioli, 
Exacum,  St.  Paulia,  or  the  fruits  of  Loasa,  rights  and  lefts  occur 


58  PROBLEMS  OF  GENETICS 

on  the  same  stem,  they  come  off  alternately.  But  if,  as  in  the 
seedlings  of  Barley  the  twist  of  the  first  leaf  be  examined,  it 
will  be  seen  to  be  either  a  right-  or  left-handed  screw.  An 
ear  of  barley,  say  a  two-row  barley,  is  a  definitely  symmetrical 
structure.  The  seeds  stand  in  their  envelopes  back  to  back  in 
definite  positions.  Each  has  its  organs  placed  in  perfectly 
definite  places.  //  these  seeds  were  buds  their  differentiations 
would  be  grouped  into  a  common  plan.  One  might  expect  that 
the  differentiations  of  these  embryos  would  still  fall  into  the 
pattern;  but  they  do  not,  and  so  far  as  I  have  tested  them,  any 
one  may  be  a  right  or  a  left,  just  as  each  may  carry  any  of  the 
Mendelian  allelomorphs  possessed  by  the  parent  plant,  without 
reference  to  the  differentiation  of  any  other  seed.  The  fertil- 
isation may  be  responsible,  but  our  experience  of  the  allelo- 
morphic  characters  suggest  that  the  irregularity  is  in  the  egg- 
cells  themselves.19 

Germ  cells  thus  differ  from  somatic  cells  in  the  fact  that  their 
differentiations  are  outside  the  geometrical  order  which  governs 
the  differentiation  of  the  somatic  cells.  I  can  think  of  possible 
exceptions,  but  I  have  confidence  that  the  rule  is  true  and  I 
regard  it  as  of  great  significance. 

The  old  riddle,  what  is  an  individual,  finds  at  least  a  partial 
solution  in  the  reply  that  an  individual  is  a  group  of  parts  dif- 
ferentiated in  a  geometrically  interdependent  order.  With  the 
germ-cell  a  new  geometrical  order,  with  independent  polarity  is 
almost  if  not  quite  always,  begun,  and  with  this  geometrical  inde- 
pendence the  power  of  rejuvenescence  may  possibly  be  associated. 

The  problems  thus  raised  are  unsolved,  but  they  do  not  look 
insoluble.  The  solution  may  be  nearer  than  we  have  thought. 
In  a  study  of  the  geometry  of  differentiation,  germinal  and 
somatic,  there  is  a  way  of  watching  and  perhaps  analyzing  what 
may  be  distinguished  as  the  mechanical  phenomena  of  heredity. 
If  any  one  could  in  the  cases  of  the  Picotee  and  the  Bizarre  Carna- 
tion, respectively,  detect  the  real  distinction  between  the  two 

19  Remarkable  experiments  on  this  question  have  lately  been  carried  out  by 
R.  H.  Compton  (Cantb.  Phil.  Soc.,  XV,  1910,  p.  495),  showing  that  in  a  certain 
Barley,  "Plumage  Corn,"  the  average  ratio  of  left  to  right  is  about  1.5.  A  fuller 
paper  has  since  been  published  by  Compton,  Jour.  Genetics,  1912,  II,  i,  p.  53. 


MERISTIC  PHENOMENA  59 

types  of  distribution,  he  would  make  a  most  notable  advance. 
Any  one  acquainted  with  mechanical  devices  can  construct  a 
model  which  will  reproduce  some  of  these  distinctions  more  or  less 
faithfully.  The  point  I  would  not  lose  sight  of  is  that  the  analogy 
with  such  models  must  for  a  long  way  be  a  true  and  valuable 
guide.  I  trust  that  some  one  with  the  right  intellectual  equip- 
ment will  endeavor  to  follow  this  guide ;  and  I  am  sanguine  enough 
to  think  that  a  comprehensive  study  of  the  geometrical  phenomena 
of  differentiation  will  suggest  to  a  penetrative  mind  that  critical 
experiment  which  may  one  day  reveal  the  meaning  of  spontaneous 
division,  the  mystery  through  which  lies  the  road,  perhaps  the 
most  hopeful,  to  a  knowledge  of  the  nature  of  life. 


CHAPTER  III 

SEGMENTATION,  ORGANIC  AND  MECHANICAL 

Models  may  be  and  often  have  been  devised  imitating  some 
of  the  phenomena  of  division,  but  none  of  them  have  reproduced 
the  peculiarity  which  characterises  divisions  of  living  tissues, 
that  the  position  of  chemical  differentiation  is  determined  by  those 
divisions.  For  example,  models  of  segmentation,  whether 
radial  or  linear,  may  be  made  by  the  vibration  of  plates  as  in 
the  familiar  Chladni  figures  of  the  physical  laboratory,  or  by 
the  bowing  of  a  tube  dusted  on  the  inside  with  lycopodium 
powder,  and  in  various  other  ways.  The  sand  or  the  powder 
will  be  heaped  up  in  the  nodes  or  regions  of  least  movement,  and 
the  patterns  thus  formed  reproduce  many  of  the  geometrical 
features  of  segmentation.  But  in  the  segmentations  of  living 
things  the  nodes  and  internodes,  once  determined  by  the  dividing 
forces,  would  each  become  the  seat  of  appropriate  and  distinct 
chemical  processes  leading  to  the  differentiation  of  the  parts, 
and  the  deposition  of  the  bones,  petals,  spines,  hairs,  and  other 
organs  in  relation  to  the  meristic  ground-plan.  The  "ripples"  of 
meristic  division  not  merely  divide  but  differentiate,  and  when 
a  "ripple"  forks  the  result  is  not  merely  a  division  but  a  re- 
duplication of  the  organ  through  which  the  fork  runs.  An 
example  illustrating  such  a  consequence  is  that  of  the  half-ver- 
tebrae of  the  Python.  On  the  left  side  the  vertebra  is  single 
(Fig.  7)  and  bears  a  single  rib,  but  on  the  right  side  a  division 
has  occurred  with  the  result  that  two  half-vertebrae,  each 
bearing  a  rib,  are  formed,  one  standing  in  succession  to  the 
other.  We  cannot,  indeed,  imagine  any  operation  of  physio- 
logical division  carried  out  in  such  an  organ  as  a  vertebra,  passing 
through  a  plane  at  right  angles  to  the  long  axis  of  the  body,  which 
does  not  necessarily  involve  the  further  process  of  reduplication. 

As  the  meristic  system  of  distribution  spreads  through  the 
body,  chemical  differentiations  follow  in  its  track,  with  seg- 

60 


SEGMENTATION 


61 


mentation  and  pattern  as  the  visible  result.  Could  we  analyse 
these  simultaneous  phenomena  and  show  how  it  is  that  the  places 
of  chemical  differentiation  are  determined  by  the  system  of 
division,  progress  would  then  be  rapid.  It  is  here  that  all 
speculation  fails. 


167 


FIGS.  7  and  8.  Two  examples  of  imperfect  division  in  the  vertebrae  of  a  python. 
I,  the  vertebrae  147-150  from  the  right  side,  showing  imperfect  division  between 
the  1 48th  and  I49th.  The  condition  on  the  left  side  of  this  vertebra  was  the  same. 
//,  the  dorsal  surface  of  vertebrae  165-167.  On  the  right  side  the  i66th  is  double 
and  bears  two  ribs,  but  on  the  left  side  it  is  normal  and  has  one  rib  only. 

Many  attempts  have  been  made  to  interpret  the  processes  of 
division  and  repetition,  in  terms  of  mechanics,  or  at  least  to 
refer  them  to  their  nearest  mechanical  analogies,  so  far  with 


62  PROBLEMS  OF  GENETICS 

little  success.  The  problem  is  beset  with  difficulties  as  yet 
insurmountable  and  of  these  one  must  be  especially  noticed.  In 
the  living  thing  the  process  by  which  repetition  and  patterns 
come  into  being  consists  partly  in  division  but  partly  also  in 
growth.  We  have  no  means  of  studying  the  phenomena  of 
pattern-formation  except  in  association  with  that  of  growth. 
Growth  soon  ceases  unless  division  takes  place,  and  if  growth  is 
impossible  division  soon  ceases  also.  In  consequence  of  this 
fact  that  the  final  pattern  is  partly  a  product  of  growth,  it  can 
never  be  used  as  unimpeachable  evidence  of  the  primary  geo- 
metrical relations  of  the  members  as  laid  down  in  the  divisions. 

In  the  last  chapter  in  referring  to  the  problem  of  repetition 
I  introduced  an  analogy,  comparing  the  patterns  of  the  organic 
world  with  those  produced  in  unorganised  materials  by  wave- 
motion.  In  the  preliminary  stage  of  ignorance,  having  no  more 
trustworthy  clue,  I  do  not  think  it  wholly  unprofitable  to  consider 
the  applicability  of  this  analogy  somewhat  more  fully.  It 
possesses,  as  I  hope  to  show,  at  least  so  much  validity  as  to 
encourage  the  belief  that  morphology  may  safely  discard  one 
source  of  long-standing  error  and  confusion. 

Those  who  have  studied  the  structure  of  parts  repeated  in 
series  will  have  encountered  the  old  morphological  problem  of 
"Serial  Homology,"  which  has  absorbed  so  much  of  the  attention 
of  naturalists  and  especially  of  zoologists  at  various  periods. 
This  problem  includes  two  separate  questions.  The  first  of 
these  is  the  origin  in  evolution  of  the  resemblance  between  two 
organs  occurring  in  a  repeated  series,  of  which  the  fore  and  hind 
limbs  of  Vertebrates  are  the  prerogative  instance.  From  the 
fact  that  these  resemblances  can  be  traced  very  far,  often  into 
minute  details  of  structure,  many  anatomists  have  inclined  to 
the  opinion  that  the  resemblance  must  originally  have  been  still 
more  complete,  and  that  the  two  limbs,  for  instance,  must  have 
acquired  their  present  forms  by  the  differentiation  of  two  iden- 
tical groups  of  parts. 

Similar  questions  arise  whenever  parts  are  repeated  in  series, 
whether  the  series  be  linear  or  radial,  and,  though  less  obviously, 
even  when  the  repetition  is  bilateral  only.  In  each  such  example 


SEGMENTATION  63 

the  question  arises,  is  the  resemblance  between  the  parts  the 
remains  of  a  still  closer  resemblance,  or  is  differentiation  original? 
Sometimes  the  view  that  these  parts  have  arisen  by  the  dif- 
ferentiation of  a  series  of  identical  parts  is  plausible  enough, 
as  for  example  when  the  peculiarities  of  various  appendages  of  a 
Decapod  Crustacean  are  referred  to  modifications  of  the  Phyl- 
lopod  series.  In  application  to  other  cases  however  we  soon  meet 
with  difficulty,  and  the  suggestion  that  the  segments  of  a  verte- 
brate were  originally  all  alike  is  seen  at  once  to  be  absurd,  for 
the  reason  that  a  creature  so  constituted  could  not  exist,  and  that, 
differentiation  of  at  least  one  anterior  and  one  posterior  segment, 
is  an  essential  condition  of  a  viable  organism  consisting  of  parts 
repeated  in  a  linear  series.  Between  these  two  terminal  segments 
it  is  possible  to  imagine  the  addition  of  one  segment,  or  of  a 
series  of  approximately  similar  segments;  but  when  once  it  is 
realised  that  the  terminals  must  have  been  differentiated  from 
the  beginning,  it  will  be  seen  that  the  problem  of  the  origin  of 
the  resemblance  between  segments  is  not  rendered  more  com- 
prehensible by  the  suggestion  that  even  the  intervening  members 
were  originally  alike.  Seeing  indeed  that  some  differentiation 
must  have  existed  primordially  it  is  as  easy  to  imagine  that  the 
original  body  was  composed  of  a  series  grading  from  the  condition 
of  the  anterior  segment  to  that  of  the  posterior,  as  any  other 
arrangement.  The  existence  of  a  linear  or  successive  series  in 
fact  postulates  a  polarity  of  the  whole,  and  in  such  a  system  the 
conception  of  an  ideal  segment  containing  all  the  parts  represented 
in  the  others  has  manifestly  no  place.  The  introduction  of  that 
conception  though  sanctioned  by  the  great  masters  of  com- 
parative anatomy,  has,  as  I  think,  really  delayed  the  progress  of 
a  rational  study  of  the  phenomena  of  division.  The  same  notion 
has  been  applied  to  every  class  of  repetition  both  in  animals  and 
plants,  generally  with  the  same  unhappy  results.  In  the  cruder 
forms  in  which  this  doctrine  was  taught  thirty  years  ago  it  is 
now  seldom  expressed,  but  modified  presentations  of  it  still 
survive  and  confuse  our  judgments. 

The  process  of  repetition  of  parts  in  the  bodies  of  organisms 
is  however  a  periodic  phenomenon.     This  much,  provided  we 


PROBLEMS   OF  GENETICS 


remain  free  from  prejudice  as  to  the  nature  and  causation  of  the 
period  or  rhythm,  we  may  safely  declare,  and  a  comparison  may 
thus  be  instituted  between  the  consequences  of  meristic  repetition 


FIG.  9.     Osmotic  growths  simulating  segmentation.     (After  Leduc.) 

in  the  bodies  of  living  things  and  those  repetitions  which  in 
the  inorganic  world  are  due  to  rhythmical  processes.  Of  such 
processes  there  is  a  practically  unlimited  diversity  and  we  have 
nothing  to  indicate  with  which  of  them  our  repetitions  should 
rather  be  compared. 


SEGMENTATION  65 

In  some  respects  perhaps  the  best  models  of  living  organisms 
yet  made  are  the  "osmotic  growths"  produced  by  Leduc.1 
These  curious  structures  were  formed  by  placing  a  fragment  of 
a  salt,  for  instance  calcium  chloride,  in  a  solution  of  some  col- 
loidal substance.  As  the  solid  takes  up  water  from  the  solution 
a  permeable  pellicle  or  membrane  is  formed  around  it.  The  ves- 
icle thus  enclosed  grows  by  further  absorption  of  water,  often 
extending  in  a  linear  direction,  and  in  many  examples  this  growth 
occurs  by  a  series  of  rhythmically  interrupted  extensions.  Some 
of  the  growths  thus  formed  are  remarkably  like  organic  structures, 
and  might  pass  for  a  series  of  antennary  segments  or  many  other 
organs  consisting  of  a  linear  series  of  repeated  parts.  In  admitting 
the  essential  resemblance  between  these  " osmotic  growths" 
and  living  bodies  or  their  organs  I  lay  less  stress  on  the  general  con- 
formation of  the  growths,  which  often  as  Leduc  points  out,  recall 
the  forms  of  fungi  or  hydroids,  but  rather  on  the  fact  that  the 
interruptions  in  the  development  of  these  systems  are  so  closely 
analogous  to  the  segmentations  or  repetitions  of  parts  character- 
istic of  living  things  (Fig.  9).  In  the  same  way  I  am  less  im- 
pressed by  Leduc's  models  of  Karyokinesis,  wonderful  as  they 
nevertheless  are,  for  the  division  is  here  imitated  by  putting 
separate  drops  on  the  gelatine  film.  What  we  most  want  to  know 
is  how  in  the  living  creature  one  drop  becomes  two.  The  models 
of  linear  segmentation  have  the  remarkable  merit  that  they  do  in 
some  measure  imitate  the  process  of  actual  division  or  repetition. 
So  in  a  somewhat  modified  method  Leduc,  by  causing  the  diffusion 
of  a  solution  in  a  gelatine  film,  produced  rhythmical  or  periodic 
precipitations  strikingly  reminiscent  of  various  organic  tissues, 
for  here  also  the  process  of  periodic  repetition  is  imitated  with 
success. 

It  is  a  feature  common  to  these  and  to  all  other  rhythmical 
repetitions  produced  by  purely  mechanical  forces  that  there 
is  resemblance  between  the  members  of  the  series,  and  that  this 
similarity  of  conformation  may  be  maintained  in  most  complex 
detail.  When  however  in  the  mechanical  series  some  of  the 
members  differ  from  the  rest  we  have  no  difficulty  in  recognising 

1  Stephane  Leduc,  Theorie  Physico-Chymiqite  de  la  Vie,  Paris,  1910. 
6 


66  PROBLEMS  OF  GENETICS 

that  these  differences — which  correspond  with  the  differenti- 
ations of  the  organic  series — are  due  to  special  heterogeneity  in 
the  conditions  or  in  the  materials,  and  it  never  occurs  to  us  to 
suppose  that  all  the  members  must  have  been  primordially  alike. 
For  example,  in  the  case  of  ripple-marks  on  the  sand,  which  I 
choose  as  one  of  the  most  familiar  and  obvious  illustrations 
of  a  repeated  series  due  to  mechanical  agencies,  if  we  notice 
one  ripple  different  in  form  from  those  adjacent  to  it,  we  do 
not  suppose  that  this  variation  must  have  been  brought  about  by 
deformation  of  a  ripple  which  was  at  first  formed  like  the  others, 
but  we  ascribe  it  to  a  difference  in  the  sand  at  that  point,  or  to  a 
difference  in  the  way  in  which  the  wind  or  the  tide  dealt  with  it. 
We  may  press  the  analogy  further  by  observing  that  in  as  much 
as  such  a  series  of  waves  has  a  beginning  and  an  end,  it  possesses 
polarity  like  that  of  the  various  linear  series  of  parts  in  organisms, 
and  even  the  formation  of  each  member  must  influence  the 
shape  of  its  successor.  Since  in  an  organism  the  beginning  and 
end  of  the  series  are  always  included,  some  differentiation  among 
the  repetitions  must  be  inevitable.  If  therefore  it  be  conceded, 
as  I  think  it  must,  that  segmentation  and  pattern  are  the  con- 
sequence of  a  periodic  process  we  realize  that  it  is  at  least  as 
easy  to  imagine  the  formation  of  such  a  series  of  parts  having 
family  likeness  combined  with  differentiation  as  it  would  be  to 
conceive  of  their  arising  primordially  as  a  series  of  identical  repe- 
titions. The  suggestion  that  the  likenesses  which  we  now  per- 
ceive are  the  remains  of  a  still  more  complete  resemblance 
is  a  substitution  of  a  more  complex  conception  for  a  simpler  one. 
The  other  question  raised  by  the  problem  of  Serial  Homology 
is  how  far  there  is  a  correspondence  between  individual  members 
of  series  when  the  series  differ  from  each  other  either  in  the 
number  of  parts,  or  in  the  mode  of  distribution  of  differentiation 
among  them.  Students,  for  example,  of  vertebrate  morphology 
debate  whether  the  nth  vertebra  which  carries  the  pelvic  girdle 
in  Lizard  A  is  individually  homologous  with  the  n+xth  vertebra 
which  fulfils  this  function  in  Lizard  B,or  whether  it  is  not  more 
truly  homologous  with  the  vertebra  standing  in  the  nth  ordinal 
position,  though  that  vertebra  in  Lizard  B  is  free. 


SEGMENTATION  67 

In  various  and  more  complex  aspects  the  same  question  is 
debated  in  regard  to  the  cranial  and  spinal  nerves,  the  branches 
of  the  aorta,  the  appendages  of  Arthropoda,  and  indeed  in  re- 
gard to  all  such  series  of  differentiated  parts  in  linear  or  suc- 
cessive repetition.  Persons  exercised  with  these  problems 
should  before  making  up  their  minds  consider  how  similar 
questions  would  be  answered  in  the  case  of  any  series  of  rhyth- 
mical repetitions  formed  by  mechanical  agencies.  In  the  case 
of  our  illustration  of  the  ripples  in  the  sand,  given  the  same  forces 
acting  on  the  same  materials  in  the  same  area,  the  number  of 
ripples  produced  will  be  the  same,  and  the  nth  ripple  counting 
from  the  end  of  the  series  will  stand  in  the  same  place  whenever 
the  series  is  evoked.  If  any  of  the  conditions  be  changed,  the 
number  and  shapes  can  be  changed  too,  and  a  fresh  "distribution 
of  differentiation"  created.  Stated  in  this  form  it  is  evident 
that  the  considerations  which  would  guide  the  judgment  in  the 
case  of  the  sand  ripples  are  not  essentially  different  from  those 
which  govern  the  problem  of  individual  homology  in  its  applica- 
tion to  vertebrae,  nerves,  or  digits. 

The  fact  that  the  unit  of  repetition  is  also  the  unit  of  growth 
is  the  source  of  the  obscurity  which  veils  the  process.  When  we 
compare  the  skeleton  of  a  long-tailed  monkey  with  that  of  a 
short-tailed  or  tailless  ape  we  see  at  once  how  readily  the  addi- 
tional series  of  caudal  segments  may  be  described  as  a  conse- 
quence of  the  propagation  of  the  "waves"  of  segmentation 
beyond  the  point  where  they  die  out  in  the  shorter  column,  and 
we  see  that  with  an  extension  of  the  series  of  repetitions  there  is 
growth  and  extension  of  material. 

The  considerations  which  apply  to  this  example  will  be  found 
operating  in  many  cases  of  the  variation  of  terminal  members  of 
linear  series.  Some  of  these  series,  like  the  teeth  of  the  dog, 
end  in  a  terminal  member  of  a  size  greatly  reduced  below  that  of 
the  next  to  it.  Even  when  there  is  thus  a  definite  specialisation 
of  the  last  member  of  the  series  it  not  infrequently  happens  that 
the  addition,  by  variation,  of  a  member  beyond  the  normal 
terminal,  is  accompanied  by  a  very  palpable  increase  in  size  of 
the  member  which  stands  numerically  in  the  place  of  the  normal 


68  PROBLEMS  OF  GENETICS 

terminal.2  So  also  with  variation  in  the  number  of  ribs,  when  a 
lumbar  vertebra  varies  homoeotically  into  the  likeness  of  the 
last  dorsal  and  bears  a  rib,  the  rib  placed  next  in  front  of  this, 
which  in  the  normal  trunk  is  the  last,  shows  a  definite  increase 
in  development. 

The  consequences  of  such  homoeoses  are  sometimes  very 
extensive,  involving  readjustments  of  differentiation  affecting 
a  long  series  of  members,  as  may  easily  be  seen  by  comparing 
the  vertebral  columns  of  several  individual  Sloths3  (whether 
Brady  pus  or  Choloepus)  to  take  a  specially  striking  example. 

It  may  be  urged  that  no  feature  as  yet  enables  us  to  perceive 
wherein  lies  the  primary  distinction  which  determines  such 
variation,  whether  it  is  due  to  a  difference  in  the  dividing  forces 
or  in  the  material  to  be  divided.  If  for  instance  we  were  to 
imitate  such  a  series  of  segments  by  pressing  hanging  drops  of 
a  viscous  fluid  out  of  a  paint-tube  by  successive  squeezes,  the 
number  of  times  the  tube  is  contracted  before  it  is  empty  will  give 
the  number  of  the  segments,  but  their  size  may  depend  either 
on  the  force  of  the  contractions  or  on  the  capacity  of  the  tube, 
or  on  various  other  factors.  Nevertheless  in  the  case  of  the 
variation  of  terminal  members,  whatever  be  the  nature  of  the 
rhythmical  impulse  which  produces  the  series  of  organs,  the  ele- 
vation of  the  normally  terminal  member  in  correspondence  with 
the  addition  of  another  is  what  we  should  expect. 

If  the  organism  acquired  its  full  size  first  and  the  delimitation 
of  the  parts  took  place  afterwards,  there  might  be  some  hope  that 
the  resemblance  between  living  patterns  and  those  mechanically 
caused  by  wave-motion  might  be  shown  to  be  a  consequence  of 
some  real  similarity  of  causation,  but  in  view  of  the  part  played 
by  growth,  appeal  to  these  mechanical  phenomena  cannot  be 
declared  to  have  more  than  illustrative  value.  Similarly  in  as 
much  as  living  patterns  appear,  and  almost  certainly  do  in  reality 
come  into  existence  by  a  rhythmical  process,  comparisons  of 
these  patterns  with  those  developed  in  crystalline  structures,  and 
in  the  various  fields  of  force  are,  as  it  seems  to  me,  inadmissible, 
or  at  least  inappropriate. 

2  Materials  for  the  Study  of  Variation,  No.  249,  p.  217;  and  p.  272. 

3  Materials,  p.  118. 


SEGMENTATION  69 

However  their  intermittence  be  determined,  the  rhythms  of 
division  must  be  looked  upon  as  the  immediate  source  of  those 
geometrically  ordered  repetitions  universally  characteristic  of 
organic  life.  In  the  same  category  we  may  thus  group  the  seg- 
mentation of  the  Vertebrates  and  of  the  Arthropods,  the  concen- 
tric growth  of  the  Lamellibranch  shells  or  of  Fishes'  scales,  the 
ripples  on  the  horns  of  a  goat,  or  the  skeletons  of  the  Foraminifera 
or  of  the  Heliozoa.  In  the  case  of  plant-structures  Church 4  has 
admirably  shown,  with  an  abundance  of  detail,  how  on  analysis 
the  definiteness  of  phyllotaxis  is  an  expression  of  such  rhythm  in 
the  division  of  the  apical  tissues,  and  how  the  spirals  and  "ortho- 
stichies"  displayed  in  the  grown  plant  are  its  ultimate  conse- 
quences. The  problem  thus  narrows  itself  down  to  the  question 
of  the  mode  whereby  these  rhythms  are  determined. 

It  is  natural  that  we  should  incline  to  refer  them  to  a  chemical 
source.  If  we  think  of  the  illustration  just  given,  of  the  seg- 
mentation of  a  viscous  fluid  into  drops  by  successive  contractions 
of  a  soft-walled  tube  we  can,  I  think,  conceive  of  such  rhythmic 
contractions  as  due  to  summations  of  chemical  stimuli,  somewhat 
as  are  the  beats  of  the  heart.  But  when  we  recognize  the  vast 
diversity  of  materials  the  distribution  of  which  is  determined  by 
an  ostensibly  similar  rhythmic  process  it  seems  hopeless  to  look 
forward  to  a  directly  chemical  solution.  That  the  chemical 
degradation  of  protoplasm  or  of  materials  which  it  contains  is 
the  source  of  the  energy  used  in  the  divisions  cannot  be  in  dispute, 
but  that  these  divisions  can  be  themselves  the  manifestations 
of  chemical  action  seems  in  the  highest  degree  improbable. 

We  may  therefore  insist  with  some  confidence  on  the  dis- 
tinction between  the  Meristic  and  the  substantive  constitution 
of  organisms,  between,  that  is  to  say,  the  system  according  to 
which  the  materials  are  divided  and  the  essential  composition 
of  the  materials,  conscious  of  the  fact  that  the  energy  of  division 
is  supplied  from  the  materials,  and  that  in  the  ontogeny  the 
manner  in  which  the  divisions  are  effected  must  depend  secon- 
darily on  the  nature  of  the  substances  to  be  divided.  The  me- 

4  Church,  A.  H.f  On  the  Relation  of  Phyllotaxis  to  Mechanical  Laws,  London, 
1904. 


70  PROBLEMS  OF  GENETICS 

chanical  processes  of  division  remain  a  distinguishable  group  of 
phenomena,  and  variations  in  the  substances  to  be  distributed  in 
division  may  be  independent  of  variations  in  the  system  by  which 
the  distribution  is  effected. 

Modern  genetic  analysis  supplies  many  remarkable  examples 
of  this  distinction.  When  formerly  we  compared  the  leaves  of  a 
normal  palmatifid  Chinese  Primula  with  the  pinnatifid  leaves6  of 
its  fern-leaved  variety  we  were  quite  unable  to  say  whether  the 
difference  between  the  two  types  of  leaf  was  due  to  a  difference 
in  the  material  cut  up  in  the  process  of  division  or  to  a  difference 
in  that  process  itself.  Knowledge  that  the  distinction  is  deter- 
mined by  a  single  segregable  factor  tends  to  prove  that  the 
critical  difference  is  one  of  substance.  So  also  in  the  Silky  fowl 
we  know  that  the  condition  of  its  feathers  is  due  to  the  absence 
of  some  one  factor  present  in  the  normal  form.  We  may  con- 
ceive such  differences  as  due  to  change  of  form  in  the  successive 
"waves"  of  division,  but  we  cannot  yet  imagine  segregation 
otherwise  than  as  acting  by  the  removal  or  retention  of  a  material 
element.  Future  observation  by  some  novel  method  may  suggest 
some  other  possibility,  but  such  cases  bring  before  us  very  clearly 
the  difficulties  by  which  the  problem  is  beset. 

In  another  region  of  observation  phenomena  occur  which  as 
it  seems  to  me  put  it  beyond  question  that  the  meristic  forces  are 
essentially  independent  of  the  materials  upon  which  they  act, 
save,  in  the  remoter  sense,  in  so  far  as  these  materials  are  the 
sources  of  energy.  The  physiology  of  those  regenerations  and 
repetitions  which  follow  upon  mutilation  supplies  a  group  of 
facts  which  both  stimulate  and  limit  speculation.  No  satis- 
factory interpretations  of  these  extraordinary  occurrences  has 
ever  been  found,  but  we  already  know  enough  to  feel  sure  that 
in  them  we  are  witnessing  indications  which  should  lead  to  the 
discovery  of  the  true  mechanics  of  repetition  and  pattern. 
The  consequences  of  mutilation  in  causing  new  growth  or  perhaps 
more  strictly  in  enabling  new  growth  to  take  place,  are  such  that 
they  cannot  be  interpreted  as  responses  to  chemical  stimuli  in 

5  It  is  a  question  whether  the  dominance  of  the  palmatifid  leaf  over  the  pin- 
natifid is  not  really  an  example  of  the  dominance  of  a  lower  number  of  segmentations 


SEGMENTATION  71 

any  sense  which  the  word  chemical  at  present  connotes.  Powers 
are  released  by  mutilation  of  which  in  the  normal  conditions  of 
life  no  sign  can  be  detected.  All  who  have  tried  to  analyse  the 
phenomena  of  regeneration  are  compelled  to  have  recourse  to  the 
metaphor  of  equilibrium,  speaking  of  the  normal  body  as  in  a  state 
of  strain  or  tension  (Morgan)  which  when  disturbed  by  muti- 
lation results  in  new  division  and  growth.  The  forces  of  division 
are  inacessible  to  ordinary  means  of  stimulation.  Applications, 
for  example,  of  heat  or  of  electricty  excite  no  responses  of  a 
positive  kind  unless  the  stimuli  are  so  violent  as  to  bring  about 
actual  destruction.8  These  agents  do  not,  to  use  a  loose  expres- 
sion, come  into  touch  with  the  meristic  forces.  Changes  in  the 
chemical  environment  of  cells  may,  as  in  the  experiments  of 
Loeb  and  of  Stockard  produce  definite  effects,  but  the  facts 
suggest  that  these  effects  are  due  rather  to  alterations  in  the 
living  material  than  to  influence  exerted  directly  on  the  forces 
of  division  themselves. 

By  destruction  of  tissue  however  the  forces  both  of  growth 
and  of  division  also  may  often  be  called  into  action  with  a  re- 
sulting regeneration.  Interruption  of  the  solid  connexion 
between  the  parts  may  produce  the  same  effects,  as  for  example 
when  the  new  heads  or  tails  grow  on  the  divided  edges  of  Pla- 
narians  (Morgan),  or  when  from  each  half  embryo  partially  sepa- 
rated from  its  normally  corresponding  half,  a  new  half  is  formed 
with  a  twin  monster  as  the  result. 

Often  classed  with  regenerations  but  in  reality  quite  distinct 
from  them  are  those  special  and  most  interesting  examples 
where  the  growth  of  a  paired  structure  is  excited  by  a  simple 

over  a  higher.  From  the  uncertainty  whether  two  given  leaves  of  two  separate 
plants  are  actually  comparable  one  cannot  institute  quite  satisfactory  numerical 
comparisons,  but  I  think  the  view  that  the  "Fern"  leaf  has  more  lobes  than  an 
otherwise  similar  "Palm"  leaf  may  be  fairly  maintained.  If  this  be  admitted, 
the  "Palm"  leaf  represents  the  dominant  low  number  and  its  round  shape  is  a 
consequence  of  the  greater  powers  of  growth  which  are  so  often  possessed  by  the 
members  of  a  shorter  series. 

6  It  is  perhaps  of  importance  to  remember  that  in  certain  species  of  bacteria 
(e.  g.  Bacillus  Anthracis)  division  may  cease  where  the  organism  is  cultivated  under 
certain  artificial  conditions  though  growth  continues.  In  this  way  very  long  un- 
segmented  threads  are  produced. 


72  PROBLEMS  OF  GENETICS 

wound.  Some  of  the  best  known  of  these  instances  are  presented 
by  the  paired  extra  appendages  of  Insects  and  Crustacea.  Some 
years  ago  I  made  an  examination  of  all  the  examples  of  such 
monstrosities  to  which  access  was  to  be  obtained,  and  it  was  with 
no  ordinary  feeling  of  excitement  that  I  found  that  these  super- 
numerary structures  were  commonly  disposed  on  a  recognizable 
geometrical  plan,  having  definite  spatial  relations  both  to  each 
other  and  to  the  normal  limb  from  which  they  grew.  The  more 
recent  researches  of  Tornier7  and  especially  his  experiments  on 
the  Frog  have  shown  that  a  cut  into  the  posterior  limb-bud 
induces  the  outgrowth  of  such  a  pair  of  limbs  at  the  wounded 
place.  Few  observations  can  compare  with  this  in  novelty  or 
significance;  and  though  we  cannot  yet  interpret  these  phenomena 
or  place  them  in  their  proper  relations  with  normal  occurrences, 
we  feel  convinced  that  here  is  an  observation  which  is  no  mere 
isolated  curiosity  but  a  discovery  destined  to  throw  a  new  light 
on  biological  mechanics.  The  supernumerary  legs  of  the  Frog 
are  evidently  grouped  in  a  system  of  symmetry  similar  to  that 
which  those  of  the  Arthropods  exhibit,  and  though  in  Arthropods 
paired  repetitions  have  not  been  actually  produced  by  injury 
under  experimental  conditions  we  need  now  have  no  hesitation 
in  referring  them  to  these  causes  as  Przibram  has  done. 

At  this  point  some  of  the  special  features  of  the  super- 
numerary appendages  become  important.  First  they  may  arise 
at  any  point  on  the  normal  limb,  being  found  in  all  situations 
from  the  base  to  the  apex.  Nor  are  they  limited  as  to  the  surface 
from  which  they  spring,  arising  sometimes  from  the  dorsal, 
anterior,  ventral,  or  posterior  surfaces,  or  at  points  intermediate 
between  these  principal  surfaces. 

With  rare  and  dubious  exceptions,  the  parts  which  are  con- 
tained in  these  extra  appendages  are  only  those  which  lie  periph- 
eral to  their  point  of  origin.  Thus  when  the  point  of  origin  is 
in  the  apical  joint  of  the  tarsus,  the  extra  growth  if  completely 
developed  consists  of  a  double  tarsal  apex  bearing  two  pairs 
of  claws.  If  they  arise  from  the  tibia,  two  complete  tarsi  are 

*  Arch.  /.  Entwm.,  XX,  1905,  p.  76;  Sitzungsb.  d.  Ges.  Naturf.,  Berlin,  1907, 
p.  41,  etc. 


SEGMENTATION 


73 


added.  If  they  spring  from  the  actual  base  of  the  appendage 
then  two  complete  appendages  may  be  developed  in  addition  to 
the  normal  one.  We  must  therefore  conclude  that  in  any  point 
on  a  normal  appendage  the  power  exists  which,  if  released,  may 
produce  a  bud  containing  in  it  a  paired  set  of  the  parts  peripheral 
to  this  point. 

Next  the  geometrical  relations  of  the  halves  of  the  super- 
numerary pair  are  determined  by  the  position  in  which  they  stand 


DA 


DAA 


FIG.  n.  Diagrams  of  the  geometrical  relations  which  are  generally  exhibited 
by  extra  pairs  of  appendages  in  Arthropoda.  The  sections  are  supposed  to  be 
those  of  the  apex  of  a  tibia  in  a  beetle.  A,  anterior,  P,  posterior,  D,  dorsal,  V, 
ventral,  if1,  MJ  are  the  imaginary  planes  of  reflexion.  The  shaded  figure  is  in 
each  case  a  limb  formed  like  that  of  the  other  side  of  the  body,  and  the  outer 
unshaded  figures  are  shaped  like  the  normal  for  the  side  on  which  the  appendages 
are.  On  the  several  radii  are  shown  the  extra  pairs  in  then:  several  possible  re- 
lations to  the  normal  from  which  they  arise.  The  normal  is  drawn  in  thick  lines 
in  the  center. 


74  PROBLEMS   OF   GENETICS 

in  regard  to  the  original  appendage.  These  relations  are  best 
explained  by  the  diagram  (Fig.  n),  from  which  it  will  be  seen 
that  the  two  supernumerary  appendages  stand  as  images  of  each 
other;  and,  of  them,  that  which  is  adjacent  to  the  normal  ap- 
pendage forms  an  image  of  it.  Thus  if  the  supernumerary  pair 
arise  from  a  point  on  the  dorsal  surface  of  the  normal  appendage, 
the  two  ventral  surfaces  of  the  extra  pair  will  face  each  other. 
If  they  arise  on  the  anterior  surface  of  the  normal  appendage, 
their  morphologically  posterior  surfaces  will  be  adjacent,  and  so  on. 

These  facts  give  us  a  view  of  the  relations  of  the  two  halves 
of  a  dividing  bud  very  different  from  that  which  is  to  be  derived 
from  the  exclusive  study  of  normal  structures.  Ordinary  mor- 
phological conceptions  no  longer  apply.  The  distribution  of  the 
parts  shows  that  the  bud  or  rudiment  which  becomes  the  super- 
numerary pair  may  break  or  open  out  in  various  ways  according 
to  its  relations  to  the  normal  limb.  Its  planes  of  division  are 
decided  by  its  geometrical  relations  to  the  normal  body. 

Especially  curious  are  some  of  the  cases  in  which  the  extra 
pair  are  imperfectly  formed.  The  appearance  produced  is  then 
that  of  two  limbs  in  various  stages  of  coalescence,  though  in 
reality  of  course  they  are  stages  of  imperfect  separation.  The 
plane  of  "coalescence"  may  fall  anywhere,  and  the  two  appen- 
dages may  thus  be  compounded  with  each  other  much  as  an 
object  partially  immersed  in  mercury  "compounds"  with  its 
optical  image  reflected  from  the  surface. 

Supernumerary  paired  structures  are  not  usually,  if  ever, 
formed  when  an  appendage  is  simply  amputated.  Cases  oc- 
casionally are  seen  which  nevertheless  seem  to  be  of  this  nature. 
Borradaile,8  for  example,  described  a  crab  (Cancer  pagurus) 
having  in  place  of  the  right  chela  three  small  chelae  arising  from 
a  common  base,  where  the  appearances  suggested  that  the  three 
reduced  limbs  replaced  a  single  normal  limb.  From  the  details  re- 
ported however  it  seems  still  possible  that  one  of  the  chelae 
(that  lettered  F.  I  in  Borradaile's  figure)  may  be  the  normal 
one,  and  the  other  two  an  extra  pair.  The  chela  which  I  suspect 
to  be  the  normal  is  in  several  respects  deformed  as  well  as  being 

8  Borradaile,  L.  A.,  Jour.  Marine  ZooL,  1897,  No.  8. 


SEGMENTATION  75 

reduced  in  size,  and  this  deformity  may  perhaps  have  ensued  as 
a  consequence  of  the  same  wound  which  excited  the  growth  of  the 
extra  pair.  Its  reduced  size  may  be  due  to  the  same  injury, 
which  may  quite  well  have  checked  its  growth  to  full  proportions. 

Admitting  doubt  in  these  ambiguous  cases  it  seems  to  be  a 
general  rule  that  for  the  production  of  the  extra  pair  the  normal 
limb  should  persist  in  connexion  with  the  body.  Moreover  it  is 
practically  certain  that  in  no  case  can  a  single,  viz.  an  unpaired, 
duplicate  of  the  normal  appendage  grow  from  it.  Many  examples 
have  been  described  as  of  this  nature,  but  all  of  them  may  be  with 
confidence  regarded  as  instances  of  a  supernumerary  pair  in 
which  only  the  two  morphologically  anterior  or  the  two  mor- 
phologically posterior  surfaces  are  developed.  We  have  thus 
the  paradox  that  a  limb  of  one  side  of  the  body,  say  the  right, 
has  in  it  the  power  to  form  a  pair  of  limbs,  right  and  left,  as  an 
outgrowth  of  itself,  but  cannot  form  a  second  left  limb  alone. 

A  very  interesting  question  arises  whether  it  is  strictly 
correct  to  describe  the  extra  pair  as  a  right  and  a  left,  or  whether 
they  are  not  rather  two  lefts  or  two  rights  of  which  one  is  reversed. 
This  question  did  not  occur  to  me  when  in  former  years  I  studied 
these  subjects.  It  was  suggested  to  me  by  Dr.  Przibram. 
The  answer  might  have  an  important  bearing  on  biological 
mechanics,  but  I  know  no  evidence  from  which  the  point  can 
be  determined  with  certainty.  In  order  to  decide  this  question 
it  would  be  necessary  to  have  cases  in  which  the  paired  repetition 
affected  a  limb  markedly  differentiated  on  the  two  sides  of  the 
body,  and  of  course  the  development  of  the  extra  parts  in  order 
to  be  decisive  must  be  fairly  complete.  One  example  only  is 
known  to  me  which  at  all  satisfies  these  requirements,  that  of  the 
lobster's  chela  figured  (after  Van  Beneden)  in  Materials  for  the 
Study  of  Variation,  p.  531,  Fig.  184,  III. 

Here  the  drawing  distinctly  suggests  that  one  of  the  extra 
dactylopodites,  namely  that  lettered  R,  is  differentiated  as  a 
left  and  not  merely  a  reversed  right.  For  the  teeth  on  this 
dactylopodite  are  those  of  a  cutting  claw,  not  of  a  crushing  claw, 
whereas  the  dactylopodites  R'  and  L'  bear  crushing  teeth.  The 
figure  makes  it  fairly  certain  also  that  the  limb  affected  was  a 


76  PROBLEMS  OF  GENETICS 

crushing  claw.  Accepting  this  interpretation,  we  reach  the 
remarkable  conclusion  that  the  bud  of  new  growth  consisted  of 
halves  differentiated  into  cutter  and  crusher  as  the  normal  claws 
are,  and  that  the  extra  crusher  is  geometrically  a  left  but  physi- 
ologically a  right.  Though  shaped  as  a  left  in  respect  of  the 
direction  in  which  it  points,  the  extra  crusher  is  really  an  op- 
tically reversed  right,  while  the  dactylopodite  R,  which  is 
placed  pointing  like  a  right,  is  really  a  reversed  left  (Fig.  12). 

If  these  indications  are  reliable 9  and  are  established  by  further 
observation  we  shall  be  led  to  the  conclusion  that  the  bud  which 


FIG.  12.  Right  claw  of  lobster  bearing  a  pair  of  extra  dactylopodites  (after 
van  Beneden).  The  fine  toothing  on  R  suggests  that  this  is  part  of  a  cutting 
claw,  though  the  limb  bearing  it  is  a  crusher. 

becomes  an  extra  pair  of  limbs  does  not  merely  contain  the  parts 
proper  to  the  side  on  which  it  grows,  but  is  comparable  with 
the  original  zygotic  cell,  and  consists  not  simply  of  two  halves, 
but  of  two  halves  differentiated  as  a  right  and  a  left  like  the  two 
halves  of  the  normal  body. 

Phenomena  of  this  kind,  evoked  by  mutilation  or  injury, 
together  with  the  cognate  observations  on  regeneration  throw 

9  Dr.  Przibram,  I  should  mention,  concludes  that  on  the  whole  the  facts  are 
against  this  interpretation,  but  as  more  evidence  is  certainly  required,  I  call  at- 
tention to  the  possibility. 


SEGMENTATION  77 

very  curious  lights  on  the  nature  of  living  things.  To  an  under- 
standing of  the  nature  of  the  mechanics  of  living  matter  and  its 
relation  to  matter  at  large  they  offer  the  most  hopeful  line  of 
approach.  I  allude  especially  to  the  examples  in  which  it  has 
been  established  that  the  part  which  is  produced  after  mutilation 
is  a  structure  different  from  that  which  was  removed.  The 
term  "regeneration"  was  introduced  before  such  phenomena 
were  discovered,  and  though  every  one  recognizes  its  inapplica- 
bility to  these  remarkable  cases,  the  word  still  misleads  us  by 
presenting  a  wrong  picture  to  the  mind.  The  expression  "  hetero- 
morphosis"  (Loeb)  has  been  appropriately  applied  to  various 
phenomena  of  this  kind,  and  Morgan  has  given  the  name  "mor- 
phallaxis"  to  another  group  of  cases  in  which  the  renewal  occurs 
by  the  transformation  of  a  previously  existing  part.10  But  we 
must  continually  remember  that  all  these  occurrences  which  we 
know  only  as  abnormalities  and  curiosities  must  in  reality  be 
exemplifications  of  the  normal  mechanics  of  division  and  growth. 
The  conditions  needed  to  call  them  forth  are  abnormal,  but  the 
responses  which  the  system  makes  are  evidences  of  its  normal 
constitution.  When  therefore,  for  example,  the  posterior  end 
of  a  worm  produces  a  reversed  tail  from  its  cut  end  we  have  a 
proof  that  there  must  be  in  the  normal  body  forces  ready  to 
cause  this  outgrowth.  The  new  structure  is  not  an  ill-shaped 
head-end,  for,  as  Morgan  shows,  the  nephridial  ducts  have  their 
funnels  perforating  the  segments  in  a  reversed  direction.  The 
"tension"  of  growth  is  actually  reversed.11  So  also  when  in  a 
Planarian  amputation  of  the  body  immediately  behind  the  head 
leads  to  the  formation  of  a  new  reversed  head  at  the  back  of 
the  normal  head,  while  amputation  further  back  leads  to  the 
regeneration  of  a  new  tail,  these  responses  give  indications 
of  forces  normally  present  in  the  body  of  the  Planarian.  Such 
facts  open  up  a  great  field  of  speculation  and  research.  Espe- 
cially important  it  would  be  to  determine  where  the  critical 
region  may  be  at  which  the  one  response  is  replaced  by  the 

10  Morgan,  T.  H.,  Regeneration,  1901. 

11  It  would  be  interesting  to  know  whether  growth  continues  at  the  original 
posterior  end  after  the  new  "posterior"  end  has  been  formed  in  front. 


78  PROBLEMS  OF  GENETICS 

\ 

other.     I  suppose  it  is  even  possible  that  there  is  some  neutral 
zone  in  which  neither  kind  of  response  is  made. 

Physical  parallels  to  the  phenomena  of  regeneration  are 
not  easy  to  find  and  we  still  cannot  penetrate  beyond  the  empirical 
facts.  Przibram  has  laid  stress  on  the  general  resemblance 
between  the  new  growth  of  an  amputated  part  in  an  animal  and 
the  way  in  which  a  broken  crystal  repairs  itself  when  placed  in 
the  mother-solution.  That  the  two  processes  have  interesting 
points  of  likeness  cannot  be  denied.  It  must  however  never  be 
forgotten  that  there  is  one  feature  strongly  distinguishing  the 
two;  for  I  believe  it  is  universally  recognized  by  physicists  that 
all  the  phenomena  of  geometrical  regularity  which  crystals 
display  are  ultimately  dependent  on  the  forms  of  the  particles 
of  the  crystalline  body.  This  cannot  in  any  sense  be  supposed 
to  hold  in  regard  to  protoplasm  or  its  constituents.  The  de- 
finiteness  of  crystals  is  also  an  unlikely  guide  for  the  reason  that 
it  is  absolute  and  perfect,  or  in  other  words  because  this  kind  of 
regularity  cannot  be  disturbed  at  all  without  a  change  so  great 
that  the  substance  itself  is  altered;  whereas  we  know  that  the 
forms  of  living  things  are  capable  of  such  changes,  great  and  small, 
that  we  must  regard  perfection  of  form,  whether  manifested  in 
symmetry  or  in  number,  as  an  ideal  which  will  only  be  produced 
in  the  absence  of  disturbance.  The  symmetry  of  the  living 
things  is  like  the  symmetry  of  the  concentric  waves  in  a  pool 
caused  by  a  splash.  Perfect  circles  are  made  only  in  the  imagi- 
nary case  of  mathematical  uniformity,  but  the  system  maintains 
an  approximate  symmetry  though  liable  to  manifold  deformation. 

Since  the  geometrical  order  of  the  living  body  cannot  be  a 
direct  function  of  the  materials  it  must  be  referred  to  some  more 
proximate  control.  In  renewing  a  part  the  body  must  possess 
the  power  of  seizing  particles  of  many  dissimilar  kinds,  and  whirl 
them  into  their  several  and  proper  places.  The  action  in  re- 
newal, like  that  of  original  growth,  may  be  compared — very 
crudely — with  the  action  of  a  separator  which  simultaneously 
distributes  a  variety  of  heterogeneous  materials  in  an  orderly 
fashion ;  but  in  the  living  body  the  thing  distributed  must  rather 
be  the  appetency  for  special  materials,  not  the  materials  them- 
selves. 


SEGMENTATION  79 

If  the  analogy  of  crystals  be  set  aside  and  we  seek  for  other 
parallels  to  regeneration  there  are  none  very  obvious.  I  have 
sometimes  wondered  whether  it  might  not  be  possible  to  institute 
a  fruitful  comparison  between  the  renewal  of  parts  and  the  refor- 
mation of  waves  of  certain  classes  after  obliteration.  In  several 
respects,  as  I  have  already  said,  some  curious  resemblances  with 
the  repetitions  formed  by  wave-motion  are  to  be  traced  in  our 
organic  phenomena,  and  though  admitting  that  I  cannot  develop 
these  comparisons,  I  think  nevertheless  they  may  be  worth 
bearing  in  mind.  When,  after  obliteration,  an  eddy  in  a  stream, 
or  a  ripple-mark  (a  more  complex  case  of  eddy-formation)  in 
blown  sand  is  re-formed,  we  have  an  example  in  which  pattern  is 
reconstituted  and  growth  takes  place  not  by  virtue  of  the  com- 
position of  the  materials — in  this  case  the  water  or  the  sand — 
but  by  the  way  in  which  they  are  acted  upon  by  extraneous 
forces. 

A  feature  in  the  actual  mode  by  which  ripple-marks  are 
reconstituted  may  not  be  without  interest  in  connexion  with 
our  phenomena  of  regeneration.  When,  for  example,  the  wind 
is  blowing  steadily  over  a  surface  of  fine,  dry  sand,  the  familiar 
ripple-marks  are  formed  by  a  heaping  of  the  sand  in  lines  trans- 
verse to  the  direction  of  the  wind.  The  heaping  is  due  to  the 
formation  of  eddies  corresponding  with  positions  of  instability. 
When  the  wind  is  steady  and  the  sand  homogeneous,  the  dis- 
tances between  the  ripples,  or  wave-lengths,  are  sensibly  equal. 
If  while  the  wind  continues  to  blow,  the  ripples  are  obliterated 
with  a  soft  brush  they  will  quickly  be  re-formed  over  the  whole 
area,  but  I  have  noticed  that  at  first  their  wave-length  is  ap- 
proximately half  that  of  the  ripples  in  the  undisturbed  parts  of 
the  system.12  The  normal  wave-length  is  restored  by  the  gradual 
accentuation  of  alternate  ripples.  Of  course  the  sand-ripples  are 
in  reality  slowly  travelling  forward  in  the  direction  towards 
which  the  wind  is. blowing,  and  for  this  our  living  segmentations 
afford  no  obvious  parallel,  but  the  appearances  in  the  area  of 

12  In  the  actual  case  observed,  the  ripples  unsmoothed  had  a  wave-length  of 
about  2>6  inches;  and  when  the  new  ones  were  first  formed,  there  were  about  30 
ridges  in  the  length  originally  traversed  by  15  or  16. 


8o  PROBLEMS   OF   GENETICS 

reformation,  and  especially  the  forking  of  the  old  ridges  where 
they  join  the  new  ones,  are  curiously  reminiscent  of  the  irregu- 
larities of  segmentation  seen  in  regenerated  structures.  The 
value  of  the  considerations  adduced  in  the  chapter  is,  I  admit, 
very  small.  The  utmost  that  can  be  claimed  for  them  is  that 
mechanical  segmentations,  like  those  seen  in  ripple-mark,  or 
in  Leduc's  osmotic  growths,  show  how  by  the  action  of  a  contin- 
uous force  in  one  direction,  repeated  and  serially  homologous 
divisions  can  be  produced  having  features  of  similarity  common 
to  those  repetitions  by  which  organic  forms  and  patterns  are 
characterised.  The  analogy  supplies  a  vicarious  picture  of  the 
phenomena  which  in  default  of  one  more  true  may  in  a  slight 
degree  assist  our  thoughts.  It  suggests  that  the  rhythms  of 
segmentation  may  be  the  consequence  of  a  single  force  definite 
in  direction  and  continuously  acting  during  the  time  of  growth. 
The  polarity  of  the  organism  would  thus  be  the  expression  of 
the  fact  that  this  meristic  force  is  definitely  directed  after  it  has 
once  been  excited,  and  the  reversal  seen  in  some  products  of  re- 
generation suggest  further  that  it  is  capable  of  being  reflected. 
This  polarity  cannot  be  a  property  of  the  material,  as  such, 
but  is  determined  by  a  force  acting  on  that  material,  just  as  the 
polarity  of  a  magnet  is  not  determined  by  the  arrangement  of  its 
particles,  but  by  the  direction  in  which  the  current  flows. 

To  some  it  may  appear  that  even  to  embark  on  such  discussions 
as  this  is  to  enter  into  a  perilous  flirtation  with  vitalistic  theories. 
How,  they  may  ask,  can  any  force  competent  to  produce  chem- 
ical and  geometrical  differentiation  in  the  body  be  distinguished 
from  the  "Entelechy"  of  Driesch?  Let  me  admit  that  in  this 
reflexion  there  is  one  element  of  truth.  If  those  who  proclaim 
a  vitalistic  faith  intend  thereby  to  affirm  that  in  the  processes 
by  which  growth  and  division  are  effected  in  the  body,  a  part  is 
played  by  an  orderly  force  which  we  cannot  now  translate  into 
terms  of  any  known  mechanics,  what  observant  man  is  not  a 
vitalist?  Driesch's  first  volume,  putting  as  it  does  into  intel- 
ligible language  that  positive  deduction  from  the  facts — es- 
pecially of  regeneration — should  carry  a  vivid  realisation  of  this 
truth  to  any  mind.  If  after  their  existence  is  realised,  it  is 


SEGMENTATION  81 

desired  that  these  unknown  forces  of  order  should  have  a  name, 
and  the  word  entelechy  is  proposed,  the  only  objection  I  have  to 
make  is  that  the  adoption  of  a  term  from  Aristotelian  philosophy 
carries  a  plain  hint  that  we  propose  to  relegate  the  future  study 
of  the  problem  to  metaphysic. 

From  this  implication  the  vitalist  does  not  shrink.  But 
I  cannot  find  in  the  facts  yet  known  to  us  any  justification  of  so 
hopeless  a  course.  It  was  but  yesterday  that  the  study  of 
Entwicklungsmechanik  was  begun,  and  if  in  our  slight  survey 
we  have  not  yet  seen  how  the  living  machine  is  to  be  expressed 
in  terms  of  natural  knowledge  that  is  poor  cause  for  despair. 
Driesch  sums  up  his  argument  thus:13 

"It  seems  to  me  that  there  is  only  one  conclusion  possible. 
If  we  are  going  to  explain  what  happens  in  our  harmonious- 
equipotential  sytems  by  the  aid  of  causality  based  upon  the 
constellation  of  single  chemical  factors  and  events,  there  must 
be  some  such  thing  as  a  machine.  Now  the  assumption  of  the 
existence  of  a  machine  proves  to  be  absolutely  absurd  in  the 
light  of  the  experimental  facts.  Therefore  there  can  be  neither 
any  sort  of  a  machine  nor  any  sort  of  causality  based  upon  con- 
stellation underlying  the  differentiation  of  harmonious-equipotential 
systems. 

"For  a  machine,  typical  with  regard  to  the  three  chief  dimen- 
sions of  space,  cannot  remain  itself  if  you  remove  parts  of  it 
or  if  you  rearrange  its  parts  at  will." 

To  the  last  clause  a  note  is  added  as  follows: 

"The  pressure  experiments  and  the  dislocation  experiments 
come  into  account  here;  for  the  sake  of  simplicity  they  have 
not  been  alluded  to  in  the  main  line  of  our  argument." 

I  doubt  whether  any  man  has  sufficient  knowledge  of  all 
possible  machines  to  give  reality  to  this  statement.  In  spite  also 
of  the  astonishing  results  of  experiments  in  dislocation,  doubt 
may  further  be  expressed  as  to  whether  they  have  been  tried  in 
such  variety  or  on  such  a  scale  as  to  justify  the  suggestion  that 
the  living  organism  remains  itself  if  its  parts  are  rearranged  at 

13  The  Science  and  Philosophy  of  the  Organism;  Gifford  Lectures,  1907.  London, 
1908,  p.  141. 


82  PROBLEMS  OF  GENETICS 

will.  All  we  know  is  that  it  can  "remain  itself"  when  much  is 
removed,  and  when  much  rearrangement  has  been  affected, 
which  is  a  different  thing  altogether. 

I  scarcely  like  to  venture  into  a  region  of  which  my  ignorance 
is  so  profound,  but  remembering  the  powers  of  eddies  to  re-form 
after  partial  obliteration  or  disturbance,  I  almost  wonder  whether 
they  are  not  essentially  machines  which  remain  themselves 
when  parts  of  them  are  removed. 

Real  progress  in  this  most  obscure  province  is  not  likely  to  be 
made  till  it  attracts  the  attention  of  physicists;  and  though  they 
for  long  may  have  to  forego  the  application  of  exact  quantitative 
methods,  I  confidently  anticipate  that  careful  comparison 
between  the  phenomena  of  repetition  formed  in  living  organisms 
and  the  various  kinds  of  segmentation  produced  by  mechanical 
agencies  would  be  productive  of  illuminating  discoveries. 


CHAPTER  IV 

THE   CLASSIFICATION   OF  VARIATION  AND   THE   NATURE  OF 
SUBSTANTIVE  FACTORS 

We  have  now  seen  that  among  the  normal  physiological 
processes  the  phenomena  of  division  form  a  recognisable,  and 
in  all  likelihood  a  naturally  distinct  group.  Variations  in  these 
respects  may  thus  be  regarded  as  constituting  a  special  class 
among  variations  in  general. 

The  substantive  variations  have  only  one  property  in  com- 
mon— the  negative  one  that  they  are  not  Meristic.  The  work 
of  classifying  them  and  distinguishing  them  according  to  their 
several  types  demands  a  knowledge  of  the  chemistry  of  life  far 
higher  than  that  to  which  science  has  yet  attained.  In  reference 
to  some  of  the  simplest  variations  Garrod  has  introduced  the 
appropriate  term  "Chemical  sports."  The  condition  in  man 
known  as  Alkaptonuria  in  which  the  urine  is  red  is  due  especially 
to  the  absence  of  the  enzyme  which  decomposes  the  excretory 
substance,  alkapton.  The  "  chemical  sport"  here  consists  in  the 
inability  to  break  up  the  benzene  ring.  The  chemical  feature 
which  distinguishes  and  is  the  proximate  cause  of  several  colour- 
varieties  can  now  in  a  few  cases  be  declared.  The  work  of  Miss 
Wheldale  has  shown  that  colour-varieties  may  be  produced  by 
the  absence  of  the  chromogen  compound  the  oxidation  of  which 
gives  rise  to  sap-colours,  by  differences  in  the  completeness  of 
this  process  of  oxidation,  and  by  a  process  of  reduction  super- 
vening on  or  perhaps  suppressing  the  oxidation.  Some  of  these 
processes  moreover  may  be  brought  about  by  the  combined  action 
of  two  bodies,  the  one  an  enzyme,  for  example  an  oxygenase,  and 
the  other  a  substance  regarded  as  a  peroxide,  contributing  the 
oxygen  necessary  for  the  oxidation  to  take  place.  Variation  in 
colour  may  thus  be  brought  about  by  the  addition  or  omission 
of  any  one  of  the  bodies  concerned  in  the  action. 

Similar  variations,  or  rather  similar  series  of  variations  will 

83 


84  PROBLEMS  OF  GENETICS 

undoubtedly  hereafter  be  identified  in  reference  to  all  the  various 
kinds  of  chemical  processes  upon  which  the  structure  and  func- 
tions of  living  things  depend.  The  identification  of  these  proc- 
esses and  of  the  bodies  concerned  in  them  will  lead  to  a  real 
classification  of  Substantive  Variations. 

To  forecast  the  lines  on  which  such  classification  will  proceed 
is  to  look  too  far  ahead.  We  may  nevertheless  anticipate  with 
some  confidence  that  future  analysis  will  recognise  among  the 
contributing  elements,  some  which  are  intrinsic  and  inalienable, 
and  others  which  are  extrinsic  and  superadded. 

We  already  know  that  there  may  be  such  interdependence 
among  the  substantive  characters  that  to  disentangle  them  will 
be  a  work  of  extreme  difficulty.  The  mere  fact  that  in  our 
estimation  characters  belong  to  distinct  physiological  systems  is 
no  proof  of  their  actual  independence.  In  illustration  may  be 
mentioned  the  sap-colour  in  Stocks  and  the  development  of 
hoariness  on  the  leaves  and  stems,  which  Miss  Saunders's  experi- 
ments have  shown  to  be  intimately  connected,  so  that  in  certain 
varieties  no  hoariness  is  produced  unless  the  elements  for  sap- 
colour  are  already  present  in  the  individual  plant. 

The  first  step  in  the  classification  of  substantive  variations 
is  therefore  to  determine  which  are  due  to  the  addition  of  new 
elements  or  factors,  and  which  are  produced  by  the  omission  of 
old  ones.  A  priori  there  is  no  valid  criterion  by  which  this  can 
be  known,  and  actual  experiments  in  analytical  breeding  can 
alone  provide  the  knowledge  required.  Some  very  curious  results 
have  by  this  method  been  obtained,  which  throw  an  altogether 
unexpected  light  on  these  problems.  For  example,  in  order  that 
the  remarkable  development  of  mesoblastic  black  pigment  char- 
acteristic of  the  Silky  Fowl  should  be  developed,  it  is  practically 
certain  that  two  distinct  variations  from  such  a  type  as  Callus 
bankiva  must  have  occurred.  I  assume,  as  is  reasonable,  that 
G.  bankiva  has  genetic  properties  similar  to  those  of  the  Brown 
Leghorn  breed  which  has  been  used  in  the  experiments  which  Mr. 
Punnett  and  I  have  conducted.  Callus  bankiva  was  not  available 
but  the  Brown  Leghorn  agrees  with  it  very  closely  in  colouration, 
and  probably  in  the  general  physiology  of  its  pigmentation. 


CLASSIFICATION  OF  VARIATIONS  85 

Setting  aside  the  various  structural  differences  between  the  two 
breeds,  the  Silky  is  immediately  distinguished  from  the  Leghorn 
by  the  fact  that  the  skin  of  the  whole  body  including  that  of  the 
face  and  comb  appears  to  be  of  a  deep  purplish  colour.  The 
face  and  comb  of  the  Leghorn  are  red  and  the  skin  of  the  body 
is  whitish  yellow.  On  examination  it  is  found  that  the  purple 
colour  of  the  Silky  is  in  reality  due  to  the  distribution  of  a  deep 
black  pigment  in  the  mesoblastic  membranes  throughout  the 
body.  The  somatopleura,  the  pleura,  pia  mater,  the  dermis,  and 
in  most  organs  the  connective  tissue  and  the  sheaths  of  the  blood- 
vessels, are  thus  impregnated  with  black.  No  such  pigmentation 
exists  in  the  Leghorn.  As  the  result  of  an  elaborate  series  of 
experimental  matings  we  have  proved  that  the  distinction  be- 
tween the  Leghorn  and  the  Silky  consists  primarily  in  the  fact 
that  the  Silky  possesses  a  pigment-producing  factor,  P,  which  is 
not  present  in  the  Leghorn. 

This  variation  must  undoubtedly  have  been  one  of  addition. 
But  besides  this  there  is  another  difference  of  an  altogether  dis- 
similar nature;  for  the  Brown  Leghorn  possesses  a  factor  which 
has  the  power  of  partially  or  completely  restricting  the  operation 
of  the  pigment-producing  factor,  P.  Moreover  in  respect  of 
this  pigment-restricting  factor  which  we  may  call  D,  the  sexes 
of  the  Brown  Leghorn  differ,  for  the  male  is  homozygous  or  DD, 
but  the  female  is  heterozygous,  Dd.  Thus  in  order  that  the 
black-skinned  breed  could  be  evolved  from  such  a  type  as  a 
Brown  Leghorn  it  must  be  necessary  both  that  P  should  be  added 
and  that  D  should  drop  out.  We  have  not  the  faintest  concep- 
tion of  the  process  by  which  either  of  these  events  have  come  to 
pass,  but  there  is  no  reasonable  doubt  that  in  the  evolution  of 
the  Silky  fowl  they  did  actually  happen. 

We  may  anticipate  that  numerous  interdependences  of  this 
kind  will  be  discovered. 

Before  any  indisputable  progress  can  be  made  with  the  prob- 
lem of  evolution  it  is  necessary  that  we  should  acquire  some  real 
knowledge  of  the  genesis  of  that  class  of  phenomena  which 
formed  the  subject  of  the  last  chapter.  So  long  as  the  process 
of  division  remains  entirely  mysterious  we  can  form  no  conception 


86  PROBLEMS  OF  GENETICS 

even  of  the  haziest  sort  as  to  the  nature  of  living  organisms,  or 
of  the  proximate  causes  which  determine  their  forms,  still  less 
can  we  attempt  any  answer  to  those  remoter  questions  of  origin 
and  destiny  which  form  the  subject  of  the  philosopher's  con- 
templation. It  is  in  no  spirit  of  dogmatism  that  I  have  ventured 
to  indicate  the  direction  in  which  I  look  for  a  solution,  though 
I  have  none  to  offer.  It  may  well  be  that  before  any  solution 
is  attained,  our  knowledge  of  the  nature  of  unorganised  matter 
must  first  be  increased.  For  a  long  time  yet  we  may  have  to 
halt,  but  we  none  the  less  do  well  to  prepare  ourselves  to  utilise 
any  means  of  advance  that  may  be  offered,  by  carefully  recon- 
noitering  the  ground  we  have  to  traverse.  The  real  difficulty 
which  blocks  our  progress  is  ignorance  of  the  nature  of  division, 
or  to  use  the  more  general  term,  of  repetition. 

Let  us  turn  to  the  more  familiar  problem  of  the  causes  of 
variation.  Now  since  variation  consists  as  much  in  meristic 
change  as  in  alteration  in  substance  or  material,  there  is  one 
great  range  of  problems  of  causation  from  which  we  are  as  yet 
entirely  cut  off.  We  know  nothing  of  the  causation  of  division, 
and  we  have  scarcely  an  observation,  experiment  or  surmise 
touching  the  causes  by  which  the  meristic  processes  may  be 
altered. 

Of  the  way  in  which  variations  in  the  substantive  composition 
of  organisms  are  caused  we  have  almost  as  little  real  evidence, 
but  we  are  beginning  to  know  in  what  such  variations  must  con- 
sist. These  changes  must  occur  either  by  the  addition  or  loss 
of  factors. 

We  must  not  lose  sight  of  the  fact  that  though  the  factors 
operate  by  the  production  of  enzymes,  of  bodies  on  which  these 
enzymes  can  act,  and  of  intermediary  substances  necessary  to 
complete  the  enzyme-action,  yet  these  bodies  themselves  can 
scarcely  be  themselves  genetic  factors,  but  consequences  of  their 
existence.  What  then  are  the  factors  themselves?  Whence  do 
they  come?  How  do  they  become  integral  parts  of  the  organism? 
Whence,  for  example,  came  the  power  which  is  present  in  a  White 
Leghorn  of  destroying — probably  reducing — the  pigment  in  its 
feathers?  That  power  is  now  a  definite  possession  of  the  breed, 


CLASSIFICATION  OF  VARIATIONS  87 

present  in  all  its  germ-cells,  male  and  female,  taking  part  in  their 
symmetrical  divisions,  and  passed  on  equally  to  all  as  much  as 
is  the  protoplasm  or  any  other  attribute  of  the  breed.  From  the 
body  of  the  bird  the  critical  and  efficient  substance  could  in  all 
likelihood  be  isolated  by  suitable  means,  just  as  the  glycogen 
of  the  liver  can  be.  But  even  when  this  extraction  has  been 
accomplished  and  the  reducing  body  isolated,  we  shall  know  no 
more  than  we  did  before  respecting  the  mode  by  which  the  power 
to  produce  it  was  conferred  on  the  fowl,  any  more  than  we  know 
how  the  walls  of  its  blood-vessels  acquired  the  power  to  form  a 
fibrin-ferment. 

It  is  when  the  scope  of  such  considerations  as  this  are  fully 
grasped  that  we  realise  the  fatuousness  of  the  conventional  treat- 
ment which  the  problem  of  the  causes  of  variation  commonly 
receives.  Environmental  change,  chemical  injury,  differences  in 
food  supply,  in  temperature,  in  moisture,  or  the  like  have  been 
proposed  as  "causes."  Admitting  as  we  must  do,  that  changes 
may  be  produced — usually  inhibitions  of  development — by  sub- 
jecting living  things  to  changes  in  these  respects,  how  can  we 
suppose  it  in  the  smallest  degree  likely  that  very  precise,  new, 
and  adaptative  powers  can  be  conferred  on  the  germs  by  such 
treatment?  Reports  of  positive  genetic  consequences  observed 
comparable  with  those  I  have  mentioned,  become  from  time  to 
time  current.  We  should  I  think  regard  them  with  the  gravest 
doubt.  Few,  so  far  as  I  am  aware,  have  ever  been  confirmed, 
though  clear  and  repeated  confirmation  should  be  demanded 
before  we  suffer  ourselves  at  all  to  build  upon  such  evidence. 
In  a  subsequent  chapter  some  of  these  cases  will  be  considered  in 
detail. 

In  no  class  of  cases  would  the  transmission  of  an  acquired 
character  superficially  appear  so  probable  as  in  those  where  power 
of  resisting  the  attack  of  a  pathogenic  organism  is  acquired  in 
the  lifetime  of  the  zygote.  The  possession  of  such  a  power  is 
moreover  a  distinction  comparable  with  those  which  differentiate 
varieties  and  species.  It  is  due  to  the  development  in  the  blood 
of  specific  substances  which  pervade  the  whole  fluid.  This 
development  is  exactly  one  of  those  "appropriate  responses  to 


88  PROBLEMS  OF  GENETICS 

stimuli"  which  naturalists  who  incline  to  regard  adaptation  as 
a  direct  consequence  of  an  environmental  influence  might  most 
readily  invoke  as  an  illustration  of  their  views.  And  yet  all 
evidence  is  definitely  unfavourable  to  the  suggestion  of  an 
inheritance  of  the  acquired  power  of  resistance.  Such  change  as 
can  be  perceived  in  the  virulence  of  the  attacks  on  successive 
generations  may  be  most  easily  regarded  as  due  to  the  exter- 
mination of  the  more  susceptible  strains,  and  perhaps  in  some 
measure  to  variation  in  the  invading  organisms  themselves,  an 
"acquired  character"  of  quite  different  import. 

The  specific  "anti-body"  may  have  been  produced  in  re- 
sponse to  the  stimulus  of  disease,  but  the  power  to  produce  it 
without  this  special  stimulus  is  not  included  in  the  germ-cells 
any  more  than  a  pigment.  All  that  they  bear  is  the  power  to 
produce  the  anti-bodies  when  the  stimulus  is  applied. 

If  we  could  conceive  of  an  organism  like  one  of  those  to 
which  disease  may  be  due  becoming  actually  incorporated  with 
the  system  of  its  host,  so  as  to  form  a  constituent  of  its  germ-cells 
and  to  take  part  in  the  symmetry  of  their  divisions,  we  should 
have  something  analogous  to  the  case  of  a  species  which  acquires 
a  new  factor  and  emits  a  dominant  variety.  When  we  see  the 
phenomenon  in  this  light  we  realise  the  obscurity  of  the  problem. 
The  appearance  of  recessive  varieties  is  comparatively  easy  to 
understand.  All  that  is  implied  is  the  omission  of  a  constituent. 
How  precisely  the  omission  is  effected  we  cannot  suggest,  but 
it  is  not  very  difficult  to  suppose  that  by  some  mechanical  fault 
of  cell-division  a  power  may  be  lost.  Such  variation  by  unpack- 
ing, or  analysis  of  a  previously  existing  complex,  though  unac- 
countable, is  not  inconceivable.  But  whence  come  the  new 
dominants?  Whether  we  imagine  that  they  are  created  by  some 
rearrangement  or  other  change  internal  to  the  organism,  or 
whether  we  try  to  conceive  them  as  due  to  the  assumption  of 
something  from  without  we  are  confronted  by  equally  hopeless 
difficulty. 

The  mystery  of  the  origin  of  a  dominant  increases  when  it 
is  realised  that  there  is  scarcely  any  recent  and  authentic  account 
of  such  an  event  occurring  under  critical  observation,  which  can 


CLASSIFICATION  OF  VARIATIONS  89 

be  taken  as  a  basis  for  discussion.  The  literature  of  horticulture 
for  example  abounds  in  cases  alleged,  but  I  do  not  think  anyone 
can  produce  an  illustration  quite  free  from  doubt.  Such  evidence 
is  usually  open  to  the  suspicion  that  the  plant  was  either  intro- 
duced by  some  accident,  or  that  it  arose  from  a  cross  with  a  pre- 
existing dominant,  or  that  it  owed  its  origin  to  the  meeting  of 
complementary  factors.  In  medical  literature  almost  alone  how- 
ever, there  are  numerous  records  of  the  spontaneous  origin  of 
various  abnormal  conditions  in  man  which  habitually  behave 
as  dominants,  and  of  the  authenticity  of  some  of  these  there 
can  be  no  doubt. 

When  we  know  that  such  conditions  as  hereditary  cataract 
or  various  deformities  of  the  fingers  behave  as  dominants,  we 
recognize  that  those  conditions  must  be  due  to  the  addition  of 
some  element  to  the  constitution  of  the  normal  man.  In  the 
collections  of  pedigrees  relating  to  such  pathological  dominants 
there  are  usually  to  be  found  alleged  instances  of  the  origin  of  the 
condition  de  novo.  Not  only  do  these  records  occur  with  such 
frequency  that  they  cannot  be  readily  set  aside  as  errors,  but  from 
general  considerations  it  must  be  obvious  that  as  these  mal- 
formations are  not  common  to  normal  humanity  they  must  at 
some  moment  of  time  have  been  introduced.  The  lay  reader 
may  not  be  so  much  impressed  with  the  difficulty  as  we  are.  He 
is  accustomed  to  regard  the  origin  of  any  new  character  as  equally 
mysterious,  but  when  once  dominants  are  distinguished  from 
recessives  the  problem  wears  a  new  aspect.  Thus  the  appearance 
of  high  artistic  gifts,  whether  as  an  attribute  of  a  race  or  as  a 
sporadic  event  among  the  children  of  parents  destitute  of  such 
faculties,  is  not  very  surprising,  for  we  feel  fairly  sure  that  the 
faculty  is  a  recessive,  due  to  the  loss  of  a  controlling  or  inhibiting 
factor;  but  the  de  novo  origin  of  brachydactylous  fingers  in  a 
child  of  normal  parents  is  of  quite  a  different  nature,  and  must 
indicate  the  action  of  some  new  specific  cause. 

Whether  such  evidence  is  applicable  to  the  general  problem 
of  evolution  may  with  some  plausibility  be  questioned;  but 
there  is  an  obvious  significance  in  the  fact  that  it  is  among  these 
pathological  occurrences  that  we  meet  with  phenomena  most 


90  PROBLEMS  OF  GENETICS 

nearly  resembling  the  spontaneous  origin  of  dominant  factors, 
and  I  cannot  see  such  pedigrees  as  these  without  recalling  Vir- 
chow's  aphorism  that  every  variation  owes  its  origin  to  some 
pathological  accident.  In  the  evolution  of  domestic  poultry, 
if  Callus  bankiva  be  indeed  the  parent  form  of  all  our  breeds, 
at  least  some  half  dozen  new  factors  must  have  been  added 
during  the  process.  In  bankiva  there  is,  for  example,  no  factor 
for  rose  comb,  pea  comb,  barring  on  the  feathers,  or  for  the 
various  dominant  types  of  dark  plumage.  Whence  came  all 
these?  It  is,  I  think,  by  no  means  impossible  that  some  other 
wild  species  now  extinct  did  take  part  in  the  constitution  of 
domestic  poultry.  It  seems  indeed  to  me  improbable  that  the 
heavy  breeds  descend  from  bankiva.  Both  in  regard  to  domestic 
races  of  fowls,  pigeons,  and  some  other  forms,  the  belief  in  origin 
within  the  period  of  human  civilization  from  one  simple  primi- 
tive wild  type  seems  on  a  balance  of  probabilities  insecurely 
founded,  but  allowing  something  for  multiplicity  of  origin  we 
still  fall  far  short  of  the  requisite  total  of  factors.  Elements 
exist  in  our  domesticated  breeds  which  we  may  feel  with  con- 
fidence have  come  in  since  their  captivity  began.  Such  ele- 
ments in  fowls  are  dominant  whiteness,  extra  toe,  feathered 
leg,  frizzling,  etc.,  so  that  even  hypothetical  extension  of  the 
range  of  origin  is  only  a  slight  alleviation  of  the  difficulty. 

Somehow  or  other,  therefore,  we  must  recognize  that  dominant 
factors  do  arise.  Whether  they  are  created  by  internal  change, 
or  whether,  as  seems  to  me  not  wholly  beyond  possibility,  they 
obtain  entrance  from  without,  there  is  no  evidence  to  show. 
If  they  were  proved  to  enter  from  without,  like  pathogenic 
organisms,  we  should  have  to  account  for  the  extraordinary 
fact  that  they  are  distributed  with  fair  constancy  to  half  the 
gametes  of  the  heterozygote. 

In  proportion  as  the  nature  of  dominants  grows  more  clear 
so  does  it  become  increasingly  difficult  to  make  any  plausible 
suggestion  as  to  their  possible  derivation.  On  the  other  hand 
the  origin  of  a  recessive  variety  by  the  loss  of  a  factor  is  a  process 
so  readily  imagined  that  our  wonder  is  rather  that  the  phenom- 
enon is  not  observed  far  more  often.  Some  slip  in  the  accurate 


CLASSIFICATION  OF  VARIATIONS  91 

working  of  the  mechanical  process  of  division,  and  a  factor 
gets  left  out,  the  loss  being  attested  by  the  appearance  of  a 
recessive  variety  in  some  subsequent  generation. 

Consistently  with  this  presentation  of  the  facts  we  find  that, 
as  in  our  domesticated  animals  and  plants,  a  diversity  of  recessives 
may  appear  within  a  moderately  short  period,  and  that  when 
variations  come  they  often  do  not  come  alone.  Witness  the 
cultural  history  of  the  Sweet  Pea,  Primula  Sinensis,  Primula 
obconica,  Nemesia  strumosa  and  many  such  examples  in  which 
variation  when  it  did  come  was  abundant.  The  fact  cannot 
be  too  often  emphasized  that  in  the  vast  proportion  of  these 
examples  of  substantive  variation  under  domestication,  as  well 
as  of  substantive  variation  in  the  natural  state,  the  change 
has  come  about  by  omission,  not  by  addition.  To  take,  for 
example,  the  case  of  the  Potato,  in  which  so  many  spontaneous 
bud-variations  have  been  recorded,  East  after  a  careful  study 
of  the  evidence  has  lately  declared  his  belief  that  all  are  of  this 
nature,  and  the  opinion  might  be  extended  to  many  other  groups 
of  cases  whether  of  bud  or  seminal  variation.  Morgan  draws 
the  same  conclusion  in  reference  to  the  many  varieties  he  has 
studied  in  Drosophila. 

In  the  Sweet  Pea,  a  form  which  is  beyond  suspicion  of  having 
been  crossed  with  anything  else,  and  has  certainly  produced 
all  the  multitude  of  types  which  we  now  possess  by  variations 
from  one  wild  species,  there  is  only  one  character  of  the  modern 
types  which  could,  with  any  plausibility,  be  referred  to  a  factor 
not  originally  forming  part  of  the  constituents  of  the  wild  species. 
This  is  the  waved  edge,  so  characteristic  of  the  "Spencer'* 
varieties;  for  the  cross  between  a  smooth-edged  and  a  waved 
type  gives  an  intermediate  not  unfrequently.  Nevertheless 
there  is  practically  no  doubt  that  this  is  merely  an  imperfection 
in  the  dominance  of  the  smooth  edge,  and  we  may  feel  sure 
that  any  plant  homozygous  for  smooth  edge  would  show  no  wave 
at  all.  Hence  it  is  quite  possible  that  even  the  appearance  of 
the  original  waved  type,  Countess  Spencer,  was  due  to  the  loss 
of  one  of  the  factors  for  smooth  edge  at  some  time  in  the  history 
of  the  Sweet  Pea. 


92  PROBLEMS  OF  GENETICS 

In  the  case  of  the  Chinese  Primrose  (Primula  Sinensis)  one 
dominant  factor  has  been  introduced  in  modern  times,  probably 
within  the  last  six  years  at  most.  This  is  the  factor  which 
causes  suppression  of  the  yellow  eye,  giving  rise  to  the  curious 
type  known  as  "  Queen  Alexandra."  Mr.  R.  P.  Gregory's 
experiments  proved  that  this  was  a  very  definite  dominant,  and 
the  element  responsible  for  this  development  is  undoubtedly  an 
addition  to  the  original  ingredient-properties,  with  which  the 
species  wras  endowed.  Unfortunately,  as  happens  in  almost  every 
case  of  the  kind,  the  origin  of  this  important  novelty  appears 
to  be  lost.  Its  behaviour,  however,  when  crossed  with  various 
other  types  is  that  of  a  simple  dominant  giving  an  ordinary  3  :  I 
ratio.  There  is  therefore  no  real  doubt  that  it  came  into  exist- 
ence by  the  definite  addition  of  a  new  factor,  for  if  it  was  simply 
a  case  of  the  appearance  of  a  new  character  made  by  combination 
of  two  previously  existing  complementary  factors  we  should 
expect  that  when  Queen  Alexandra  was  self-fertilised  a  9  :  7 
ratio  would  be  a  fairly  common  result,  which  is  not  in  practice 
found. 

In  Oenothera  Gates l  has  observed  the  appearance,  in  a  large 
sowing  of  about  1 ,000  Oenothera  rubrinervis,  of  a  single  individual 
having  considerably  more  red  pigment  in  the  calyx  than  is  usual 
in  rubrinervis.  The  whole  of  the  hypanthium  in  the  flowers  of 
this  plant  was  red  instead  of  green  as  in  rubrinervis,  and  the 
whole  of  the  sepals  were  red  in  the  bud-stage,  except  for  small 
green  areas  at  the  base.  This  type  behaved  as  a  dominant  over 
rubrinervis,  but  so  far  a  pure-breeding  individual  was  not  found. 
Admittedly  the  variation  of  this  plant  from  the  type  of  rubrinervis 
can  be  represented  as  one  of  degree,  though  there  is  a  very 
sensible  gap  in  the  series  between  the  new  form  which  Gates 
names  "rubricalyx"  and  the  reddest  rubrinervis  seen  in  his  cul- 
tures. It  must  certainly  be  recognised  as  a  new  dominant. 
Gates,  rightly  as  I  consider,  regards  the  distinction  between 
rubrinervis  and  rubricalyx  as  a  quantitative  one,  and  the  same 
remark  applies  to  certain  other  types  differing  in  the  amount  of 
anthocyanin  which  they  produce.  I  do  not  understand  the  argu- 

i  Gates,  R.  R.,  Zts.  f.  Abstammungslehre,  1911,  IV,  pp.  341  and  361. 


CLASSIFICATION  OF  VARIATIONS  93 

ment  which  Gates  introduces  to  the  effect  that  the  difference 
between  such  quantitative  types  cannot  be  represented  in  terms 
of  presence  and  absence.  We  are  quite  accustomed  to  the  fact 
that  in  the  rabbit  self-colour  segregates  from  the  Dutch-marked 
type.  These  two  types  differ  in  a  manner  which  we  may  reason- 
ably regard  as  quantitative.  It  is  no  doubt  possible  that  the 
self-coloured  type  contains  an  ingredient  which  enables  the  colour 
to  spread  over  the  whole  body,  but  it  is,  I  think,  perhaps  more 
easy  to  regard  the  Dutch  type  as  a  form  from  which  a  part  of 
the  colour  is  absent.  It  may  be  spoken  of  in  terms  I  have  used, 
as  a  subtraction-stage  in  colour.  Following  a  similar  method  we 
may  regard  rubricalyx  as  an  addition-stage  in  colour- variation. 
The  fact  that  crosses  between  rubrinervis,  or  rubricalyx  and 
Lamarckiana  give  a  mixture  of  types  in  Fi,  does  not  I  think  show, 
as  Gates  declares,  that  there  is  any  system  here  at  work  to 
which  a  factorial  or  Mendelian  analysis  does  not  apply;  but  that 
question  may  be  more  fitly  discussed  in  connexion  with  the  other 
problems  raised  by  the  behaviour  of  Oenothera  species  in  their 
crosses. 

I  do,  however,  feel  that,  interesting  as  this  case  must  be 
admitted  to  be,  we  cannot  quite  satisfactorily  discuss  it  as  an 
illustration  of  the  de  novo  origin  of  a  dominant  factor.  The 
difference  between  the  novelty  and  the  type  is  quantitative,  and 
it  is  not  unreasonable  to  think  of  such  a  difference  being  brought 
about  by  some  "pathological  accident"  in  a  cell-division. 

Recognition  of  the  distinction  between  dominant  and  reces- 
sive characters  has,  it  must  be  conceded,  created  a  very  serious 
obstacle  in  the  way  of  any  rational  and  concrete  theory  of  evolu- 
tion. While  variations  of  all  kinds  could  be  regarded  as  mani- 
festations of  some  mysterious  instability  of  organisms  this  diffi- 
culty did  not  occur  to  the  mind  of  evolutionists.  To  most  of 
those  who  have  taken  part  in  genetic  analysis  it  has  become  a 
permanent  and  continual  obsession.  With  regard  to  the  origin 
of  recessive  variations,  there  is,  as  we  have  seen,  no  special 
difficulty.  They  are  negative  and  are  due  to  absences,  but  as 
soon  as  it  is  understood  that  dominants  are  caused  by  an  addition 
we  are  completely  at  a  loss  to  account  for  their  origin,  for  we 


94  PROBLEMS  OF  GENETICS 

cannot  surmise  any  source  from  which  they  may  have  been 
derived.  Just  as  when  typhoid  fever  breaks  out  in  his  district 
the  medical  officer  of  health  knows  for  certain  that  the  bacillus  of 
typhoid  fever  has  by  some  means  been  brought  into  that  district 
so  do  we  know  that  when  first  dominant  white  fowls  arose  in  the 
evolution  of  the  domestic  breeds,  by  some  means  the  factor  for 
dominant  whiteness  got  into  a  bird,  or  into  at  least  one  of  its 
germ-cells.  Whence  it  came  we  cannot  surmise. 

Whether  we  look  to  the  outer  world  or  to  some  rearrangement 
within  the  organism  itself,  the  prospect  of  finding  a  source  of 
such  new  elements  is  equally  hopeless. 

Leaving  this  fundamental  question  aside  as  one  which  it  is 
as  yet  quite  unprofitable  to  discuss,  we  are  on  safe  ground  in 
foreseeing  that  the  future  classification  of  substantive  variations, 
which  genetic  research  must  before  long  make  possible,  will  be 
based  on  a  reference  to  the  modes  of  action  of  the  several  factors. 
Some  will  be  seen  to  produce  their  effects  by  oxidation,  some  by 
reduction,  some  by  generating  substances  of  various  types, 
sugars,  enzymes,  activators,  and  so  forth.  It  may  thus  be 
anticipated  that  the  relation  of  varieties  to  each  other  and  to 
types  from  which  they  are  derived  will  be  expressible  in  terms 
of  definite  synthetical  formulae.  Clearly  it  will  not  for  an  in- 
definite time  be  possible  to  do  this  in  practice  for  more  than  a 
few  species  and  for  characters  especially  amenable  to  experi- 
mental tests,  but  as  soon  as  the  applicability  of  such  treatment 
is  generally  understood  the  influence  on  systematics  must  be 
immediate  and  profound,  for  the  nature  of  the  problem  will  at 
length  be  clear  and,  though  the  ideal  may  be  unattainable,  its 
significance  cannot  be  gainsaid. 


Note. — With  hesitation  I  allow  this  chapter  to  appear  in 
the  form  in  which  it  was  printed  a  year  ago,  but  in  passing  it  for 
the  press  after  that  interval  I  feel  it  necessary  to  call  attention 
to  a  possible  line  of  argument  not  hitherto  introduced. 

In  all  our  discussions  we  have  felt  justified  in  declaring  that 
the  dominance  of  any  character  indicates  that  some  factor  is 


CLASSIFICATION  OF  VARIATIONS  95 

present  which  is  responsible  for  the  production  of  that  character. 
Where  there  is  no  definite  dominance  and  the  heterozygote  is 
of  an  intermediate  nature  we  should  be  unable  to  declare  on 
which  side  the  factor  concerned  was  present  and  from  which  side 
it  was  absent.  The  degree  of  dominance  becomes  thus  the 
deciding  criterion  by  which  we  distinguish  the  existence  of  factors. 
But  it  should  be  clearly  realized  that  in  any  given  case  the  argu- 
ment can  with  perfect  logic  be  inverted.  We  already  recognize 
cases  in  which  by  the  presence  of  an  inhibiting  factor  a  character 
may  be  suppressed  and  purely  as  a  matter  of  symbolical  expres- 
sion we  might  apply  the  same  conception  of  inhibition  to  any 
example  of  factorial  influence  whatever.  For  instance  we  say 
that  in  as  much  as  two  normal  persons  do  not  have  brachydacty- 
lous  children,  there  must  be  some  factor  in  these  abnormal  persons 
which  causes  the  modification.  Our  conclusion  is  based  on  the 
observed  fact  that  the  modification  is  a  dominant.  But  it  may 
be  that  normal  persons  are  homozygous  in  respect  of  some  factor 
N,  which  prevents  the  appearance  of  brachydactyly,  and  that  in 
any  one  heterozygous,  Nn,  for  this  inhibiting  factor,  brachy- 
dactyly can  appear.  Similarly  the  round  pea  we  say  contains 
R,  a  factor  which  confers  this  property  of  roundness,  without 
which  its  seeds  would  be  wrinkled.  But  here  we  know  that  the 
wrinkled  seed  is  in  reality  one  having  compound  starch-grains, 
and  that  the  heterozygote,  though  outwardly  round  enough,  is 
intermediate  in  that  starch-character.  If  we  chose  to  say  that 
the  compoundness  of  the  grains  is  due  to  a  factor  C  and  that  two 
doses  of  it  are  needed  to  make  the  seed  wrinkled,  I  know  no 
evidence  by  which  such  a  thesis  could  be  actually  refuted.  That 
such  reasoning  is  seemingly  perverse  must  be  conceded;  but 
when  we  consider  the  extraordinary  difficulties  which  beset 
any  attempt  to  conceive  the  mode  of  origin  of  a  new  dominant 
factor,  we  are  bound  to  remember  that  there  is  this  other  line  of 
argument  which  avoids  that  difficulty  altogether.  In  the  case  of 
the  "  Alexandra  "-eye  in  Primula,  or  the  red  calyx  in  Gates's 
Oenothera,  inverting  the  reasoning  adopted  in  the  text,  we  may 
see  that  only  the  Primula  homozygous  for  the  yellow  eye  can 
develop  it  and  that  two  doses  of  the  factor  for  the  rubrinervis 


96  PROBLEMS  OF  GENETICS 

calyx  are  required  to  prevent  that  part  of  the  plant  from  being 
red. 

We  may  proceed  further  and  extend  this  mode  of  reasoning  to 
all  cases  of  genetic  variation,  and  thus  conceive  of  all  alike  as 
due  to  loss  of  factors  present  in  the  original  complex.  Until  we 
can  recognize  factors  by  means  more  direct  than  are  provided  by 
a  perception  of  their  effects,  this  doubt  cannot  be  positively 
removed.  For  all  practical  purposes  of  symbolic  expression  we 
may  still  continue  to  use  in  our  analyses  the  modes  of  representa- 
tion hitherto  adopted,  but  we  must  not,  merely  on  the  ground  of 
its  apparent  perversity,  refuse  to  admit  that  the  line  of  argument 
here  indicated  may  some  day  prove  sound. 


CHAPTER  V 

THE   MUTATION   THEORY 

When  with  the  thoughts  suggested  in  the  last  chapter  we 
contemplate  the  problem  of  Evolution  at  large  the  hope  at  the 
present  time  of  constructing  even  a  mental  picture  of  that  process 
grows  weak  almost  to  the  point  of  vanishing.  We  are  left 
wondering  that  so  lately  men  in  general,  whether  scientific  or 
lay,  were  so  easily  satisfied.  Our  satisfaction,  as  we  now  see, 
was  chiefly  founded  on  ignorance. 

Every  specific  evolutionary  change  must  represent  a  definite 
event  in  the  construction  of  the  living  complex.  That  event 
may  be  a  disturbance  in  the  meristic  system,  showing  itself  in 
a  change  in  the  frequency  of  the  repetitions  or  in  the  distribution 
of  differentiation  among  them,  or  again  it  may  be  a  chemical 
change,  adding  or  removing  some  factor  from  the  sum  total. 

If  an  attempt  be  made  to  apply  these  conceptions  to  an  actual 
series  of  allied  species  the  complexity  of  the  problem  is  such  that 
the  mind  is  appalled.  Ideas  which  in  the  abstract  are  appre- 
hended and  accepted  with  facility  fade  away  before  the  concrete 
case.  It  is  easy  to  imagine  how  Man  was  evolved  from  an 
Amoeba,  but  we  cannot  form  a  plausible  guess  as  to  how  Veronica 
agrestis  and  Veronica  polita  were  evolved,  either  one  from  the 
other,  or  both  from  a  common  form.  We  have  not  even  an 
inkling  of  the  steps  by  which  a  Silver  Wyandotte  fowl  descended 
from  Callus  Bankiva,  and  we  can  scarcely  even  believe  that  it 
did.  The  Wyandotte  has  its  enormous  size,  its  rose  comb,  its 
silver  lacing,  its  tame  spirit,  and  its  high  egg  production.  The 
tameness  and  the  high  egg  production  are  probably  enough  both 
recessives,  and  though  we  cannot  guess  how  the  corresponding 
dominant  factors  have  got  lost,  it  is  not  very  difficult  to  imagine 
that  they  were  lost  somehow.  But  the  rose  comb  and  the  silver 
colour  are  dominants.  The  heavy  weight  also  appears  in  the 
crosses  with  Leghorns,  but  we  need  not  at  once  conclude  that  it 

97 


98  PROBLEMS  OF  GENETICS 

depends  on  a  simple  dominant  factor,  because  the  big  size  of 
the  crosses  may  be  a  consequence  of  the  cross  and  may  depend  on 
other  elements. 

Now  no  wild  fowl  known  to  us  has  these  qualities.  May  we 
suppose  that  some  extinct  wild  species  had  them?  If  so,  may 
we  again  make  the  same  supposition  in  all  similar  cases?  To  do 
so  is  little  gain,  for  we  are  left  with  the  further  problem,  whence 
did  those  lost  wild  species  acquire  those  dominants?  Supposi- 
tions of  this  kind  help  no  more  than  did  the  once  famous 
conjecture  as  to  the  origin  of  living  things — that  perhaps  they 
came  to  earth  on  a  meteorite.  The  unpacking  of  an  original 
complex,  the  loss  of  various  elements,  and  the  recombination  of 
pre-existing  materials  may  all  be  invoked  as  sources  of  specific 
diversity.  Undoubtedly  the  range  of  possibilities  thus  opened 
up  is  large.  It  will  even  cover  an  immense  number  of  actual 
examples  which  in  practice  pass  as  illustrations  of  specific  dis- 
tinction. The  Indian  Rock  pigeon  which  has  a  blue  rump 
may  quite  reasonably  be  regarded  as  a  geographically  separated 
recessive  form  of  our  own  Columba  lima,  for  as  Staples-Browne 
has  shown  the  white  rump  of  livid  is  due  to  a  dominant  factor. 
The  various  degrees  to  which  the  leaves  of  Indian  Cottons  are 
incised  have,  as  Leake  says,  been  freely  used  as  a  means  of  classi- 
fication. The  diversities  thus  caused  are  very  remarkable, 
and  when  taken  together  with  diversities  in  habit,  whether 
sympodial  or  monopodial,  the  various  combinations  of  points 
of  difference  are  sufficiently  distinctive  to  justify  any  botanist 
in  making  a  considerable  number  of  species  by  reference  to  them 
alone.  Nevertheless  Leake's  work  goes  far  to  prove  that  all  of 
these  forms  represent  the  re-combinations  of  a  very  small  number 
of  factors.  The  classical  example  of  Primula  Sinensis  and  its 
multiform  races  is  in  fact  for  a  long  way  a  true  guide  as  to  the 
actual  interrelations  of  the  species  which  systematists  have 
made.  That  they  did  make  them  was  due  to  no  mistake  in 
judgment  or  in  principle,  but  simply  to  the  want  of  that  ex- 
tended knowledge  of  the  physiological  nature  of  the  specific 
cases  which  we  now  know  to  be  a  prime  necessity. 

But  will  such  analysis  cover  all  or  even  most  of  the  ordinary 


THE  MUTATION  THEORY  99 

cases  of  specific  diversity  between  near  allies?  Postponing  the 
problem  of  the  interrelations  of  the  larger  divisions  as  altogether 
beyond  present  comprehension,  can  we  suppose,  that  in  general, 
closely  allied  species  and  varieties  represent  the  various  con- 
sequences of  the  presence  or  absence  of  allelomorphic  factors 
in  their  several  combinations?  The  difficulty  in  making  a 
positive  answer  lies  in  the  fact  that  in  most  of  the  examples  in 
which  it  has  been  possible  to  institute  breeding  experiments  with 
a  view  to  testing  the  question,  a  greater  or  less  sterility  is  en- 
countered. Where,  however,  no  such  sterility  is  met  with,  as 
for  instance  in  the  crosses  made  by  E.  Baur  among  the  species 
of  Antirrhinum  there  is  every  reason  to  think  that  the  whole 
mass  of  differences  can  and  will  eventually  be  expressed  in  terms 
of  ordinary  Mendelian  factors.  Baur  has  for  example  crossed 
species  so  unlike  as  Antirrhinum  majus  and  molle,  forms  differing 
from  each  other  in  almost  every  feature  of  organisation.1  The 
Fz  generation  from  this  cross  presents  an  amazingly  motley 
array  of  types  which  might  easily  if  met  with  in  nature  be  de- 
scribed as  many  distinct  species.  Yet  all  are  fertile  and  there 
is  not  the  slightest  difficulty  in  believing  that  they  can  all  be 
reduced  to  terms  of  factorial  a'nalysis. 

If  allowance  be  made  for  the  complicating  effects  of  sterility, 
is  there  anything  which  prevents  us  from  supposing  that  such 
good  species  as  those  of  Veronica  or  of  any  other  genus  comprising 
well-defined  forms  may  not  be  similarly  related?  I  do  not  know 
any  reason  which  can  be  pointed  to  as  finally  excluding  such  a 
possibility.  Nevertheless  it  has  been  urged  with  some  plausi- 
bility that  good  species  are  distinguished  by  groups  of  differ- 
entiating characters,  whereas  if  they  were  really  related  as  the 
terms  of  a  Mendelian  F2  family  are,  we  should  expect  to  find 
not  groups  of  characters  in  association,  but  rather  series  of  forms 
corresponding  to  the  presence  and  absence  of  the  integral  factors 
composing  the  groups  of  characters.  I  am  not  well  enough 
versed  in  systematic  work  to  be  able  to  decide  with  confidence 
how  much  weight  should  be  attached  to  this  consideration.  Some 

1  See  Lotsy  and  Baur,  Rep.  Genetics  Conf.,  Paris,  1911,  pp.  416-426.  Com- 
pare Lecoq  on  Mirabilis  jalapa  X  longiflora,  Fecondation  des  Vegetaux,  1862,  p.  311. 


ioo  PROBLEMS  OF  GENETICS 

weight  it  certainly  has,  but  I  cannot  yet  regard  it  as  forming  a 
fatal  objection  to  the  application  of  factorial  conceptions  on 
the  grand  scale.  It  may  be  recalled  that  we  are  no  longer  under 
any  difficulty  in  supposing  that  differences  of  all  classes  may  be 
caused  by  the  presence  or  absence  of  factors.  It  seemed  at  first 
for  example  that  such  characters  as  those  of  leaf  shape  might 
be  too  subtle  and  complex  to  be  reducible  to  a  limited  number  of 
factors.  But  first  the  work  of  Gregory  on  Primula  Sinensis 
showed  that  several  very  distinct  types  of  leaves  were  related 
to  each  other  in  the  simplest  way.  In  that  particular  example, 
intermediates  are  so  rare  as  to  be  negligible,  but  subsequently 
Shull  dealing  with  such  a  complicated  example  as  Capsella,  and 
Leake  in  regard  to  Cottons,  both  forms  in  which  intergrades  occur 
in  abundance,  have  shown  that  a  simple  factorial  scheme  is 
applicable.  We  need  not  therefore,  to  take  an  extreme  case, 
doubt  that  if  it  were  possible  to  examine  the  various  forms  of 
fruit  seen  in  the  Squashes  by  really  comprehensive  breeding 
tests,  even  this  excessive  polymorphism  in  respect  of  structural 
features  would  be  similarly  reducible  to  factorial  order. 

It  must  always  be  remembered  also  that  in  a  vast  number 
of  cases,  nearly  allied  forms  which  are  distinct,  occupy  distinct 
ground.  Moreover,  by  whatever  of  the  many  available  mechan- 
isms that  end  be  attained,  it  is  clear  that  nature  very  often  does 
succeed  in  preventing  intercrossing  between  distinct  forms  so 
far  that  the  occurrence  of  that  phenomenon  is  a  rarity  under 
natural  conditions.  The  facts  may,  I  think,  fairly  be  summarized 
in  the  statement  that  species  are  on  the  whole  distinct  and  not 
intergrading,  and  that  the  distinctions  between  them  are  usually 
such  as  might  be  caused  by  the  presence,  absence,  or  inter-com- 
bination of  groups  of  Mendelian  factors;  but  that  they  are  so 
caused  the  evidence  is  not  yet  sufficient  to  prove  in  more  than  a 
very  few  instances. 

The  alternative,  be  it  explicitly  stated,  is  not  to  return  to  the 
view  formerly  so  widely  held,  that  the  distinctions  between 
species  have  arisen  by  the  accumulation  of  minute  or  insensible 
differences.  The  further  we  proceed  with  our  analyses  the  more 
inadequate  and  untenable  does  that  conception  of  evolutionary 


THE  MUTATION  THEORY  101 

change  become.  If  the  differences  between  species  have  not 
come  about  by  the  addition  or  loss  of  factors  one  at  a  time,  then 
we  must  suppose  that  the  changes  have  been  effected  by  even 
larger  steps,  and  variations  including  groups  of  characters,  must 
be  invoked. 

That  changes  of  this  latter  order  are  really  those  by  which 
species  arise,  is  the  view  with  which  de  Vries  has  now  made  us 
familiar  by  his  writings  on  the  Mutation  Theory.  In  so  far  as 
mutations  may  consist  in  meristic  changes  of  many  kinds  and 
in  the  loss  of  factors  it  is  unnecessary  to  repeat  that  we  have 
abundant  evidence  of  their  frequent  occurrence.  That  they  may 
also  more  rarely  occur  by  the  addition  of  a  factor  we  are,  I  think, 
compelled  to  believe,  though  as  yet  the  evidence  is  almost  en- 
tirely circumstantial  rather  than  direct.  The  evidence  for  the 
occurrence  of  those  mutations  of  higher  order,  by  which  new 
species  characterized  by  several  distinct  features  are  created, 
is  far  less  strong,  and  after  the  best  study  of  the  records  which 
I  have  been  able  to  make,  I  find  myself  unconvinced.  The  facts 
alleged  appear  capable  of  other  interpretations. 

The  most  famous  and  best  studied  examples  are  of  course  the 
forms  of  Oenothera  raised  by  de  Vries  from  Oenothera  Lamarckiana 
in  circumstances  well  known  to  all  readers  of  genetic  literature. 
Whatever  be  the  true  significance  of  these  extraordinary  "mu- 
tations" there  can  be  no  question  about  the  great  interest  which 
attaches  to  them,  and  the  historical  importance  which  they  will 
long  preserve.  Apart  also  from  these  considerations  it  is  be- 
coming more  and  more  evident  that  in  their  peculiarities  they 
provide  illustrations  of  physiological  phenomena  of  the  highest 
consequence  in  the  study  of  genetics  at  large. 

De  Vries  found,  as  is  well  known,  that  Oenothera  Lamarckiana 
gives  off  plants  unlike  itself.  These  mutational  forms  are  of 
several  distinct  and  recognizable  types  which  recur,  and  several 
of  them  breed  true  from  their  first  appearance.  The  obvious 
difficulty,  which  in  my  judgment  should  make  us  unwilling  at 
present  to  accept  these  occurrences  as  proof  of  the  genesis  of  new 
species  by  mutation,  is  that  we  have  as  yet  no  certainty  that  the 
appearance  of  the  new  forms  is  not  an  effect  of  the  recombination 


102  PROBLEMS  OF  GENETICS 

of  factors,  such  as  is  to  be  seen  in  so  many  generations  of  plants 
derived  from  a  cross  involving  many  genetic  elements.  The 
first  question  is  what  is  Oenothera  Lamarckiana?  Is  it  itself  a 
plant  of  hybrid  origin?  To  this  fundamental  question  no  satis- 
factory answer  has  yet  been  given.  All  attempts  to  find  it  as  a 
wild  plant  in  America  have  failed.  It  existed  in  Europe  in  the 
latter  half  of  the  eighteenth  century.  Whence  it  came  is  still 
uncertain,  but  the  view  that  it  came  into  existence  in  Europe  and 
perhaps  in  Paris,  seems  on  the  whole  the  most  probable.  The 
question  has  been  debated  by  Macdougal,  Gates,  and  Davis. 
From  historical  sources  there  is  little  expectation  of  further 
light.  Those  who  favour  the  notion  of  a  hybrid  origin  look  on 
Oenothera  biennis  as  one  of  the  putative  parents.  It  has  been 
conjectured  that  a  species  called  grandifiora  lately  re-discovered 
on  the  Alabama  river  was  the  other  parent.  Experiments  have 
been  instituted  by  Davis  to  discover  whether  Lamarckiana  can 
be  made  artificially  by  crossing  these  two  species.  The  results 
so  far  have  shown  that  while  plants  approximating  in  various 
respects  to  Lamarckiana  have  thus  been  produced,  none  agree 
exactly  with  that  form.  Davis,  to  whom  reference  should  be 
made  for  a  full  account  of  the  present  state  of  the  enquiry, 
points  out  that  there  are  many  strains  of  biennis  in  existence 
and  that  it  is  by  no  means  impossible  that  by  using  others  of 
these  strains  a  still  closer  approximation  can  be  made.  None 
of  Davis's  artificial  productions  as  yet  breed  at  all  true,  as 
Lamarckiana  on  the  whole  does.  In  such  a  case,  however,  where 
several  characters  are  involved,  this  is  perhaps  hardly  to  be 
expected. 

One  feature  of  the  Oenotheras  is  very  curious.  Not  only 
Lamarckiana,  but  all  the  allied  species  so  far  as  I  am  aware, 
have  a  considerable  proportion  of  bad  and  shrivelled  pollen 
grains.  This  is  undoubtedly  true  of  species  living  in  the  wild 
state  as  well  as  of  those  in  cultivation.  I  have  had  opportunities 
of  verifying  this  for  myself  in  the  United  States.  No  one  looking 
at  the  pollen  of  an  Oenothera  would  doubt  that  it  was  taken  from 
some  hybrid  plant  exhibiting  partial  sterility.  On  the  other 
hand,  it  is  difficult  to  suppose  that  numbers,  perhaps  all,  of  the 


THE  MUTATION  THEORY  103 

"  species"  of  the  genus  are  really  hybrids,  and  many  of  them  breed 
substantially  true.  I  regard  this  constant  presence  of  bad 
pollen  grains  as  an  indication  that  the  genetic  physiology  of 
Oenothera  is  in  some  way  abnormal,  and  as  we  shall  presently 
see,  there  are  several  other  signs  which  point  in  the  same  direction. 

Discussion  of  the  whole  series  of  phenomena  is  rendered 
exceedingly  difficult  first,  by  reason  of  the  actual  nature  of  the 
material.  The  characteristics  of  many  of  the  types  which  de  Vries 
has  named  are  evasive.  A  few  of  these  types,  for  instance,  gigas, 
nanella,  albida,  brevistylis,  and  perhaps  a  few  more  are  evidently 
clear  enough,  but  we  have  as  yet  no  figures  and  descriptions 
precise  enough  to  enable  a  reader  to  appreciate  exactly  the  pe- 
culiarities of  the  vast  number  of  forms  which  have  now  to  be 
considered  in  any  attempt  to  gain  a  comprehensive  view  of  the 
whole  mass  of  facts.  It  is  also  not  in  dispute  that  the  forms  are 
susceptible  of  great  variations  due  simply  to  soil  and  cultural 
influences. 

The  fact  that  no  Mendelian  analysis  has  yet  been  found 
applicable  to  this  group  of  Oenotheras  as  a  whole  is  perhaps  largely 
due  to  the  fact  that  until  recently  such  analysis  has  not  been 
seriously  attempted.  Following  the  system  which  he  had 
adopted  before  the  rediscovery  of  Mendelism,  or  at  all  events, 
before  the  development  of  that  method  of  analysis,  de  Vries  has 
freely  applied  names  to  special  combinations  of  characters  and 
has  scarcely  ever  instituted  a  factorial  analysis.  Before  we  can 
get  much  further  this  must  be  attempted.  It  may  fail,  but  we 
must  know  exactly  where  and  how  this  failure  comes  about. 
There  are  several  indications  that  such  a  recognition  of  factorial 
characters,  could  be  carried  some  way.  For  example,  the  height, 
the  size  of  the  flowers,  the  crinkling  of  the  leaves,  the  brittleness 
of  the  stems,  perhaps  even  the  red  stripes  on  stems  and  fruits, 
and  many  more,  are  all  characters  which  may  or  may  not  depend 
on  distinct  factors,  but  if  such  characters  are  really  transmitted 
in  unresolved  groups,  the  limitations  of  those  groups  should  be 
carefully  determined.  The  free  use  of  names  for  the  several 
forms,  rather  than  for  the  characters,  has  greatly  contributed 
to  deepen  the  obscurity  which  veils  the  whole  subject. 


104  PROBLEMS  OF  GENETICS 

1  do  not  mean  to  suggest  that  these  Oenotheras  follow  a  simple 
Mendelian  system.     All  that  we  know  of  them  goes  to  show  that 
there  are  curious  complications  involved.     One  of  these,  prob- 
ably the  most  important  of  all,  has  lately  been  recognized  by 
de  Vries  himself,  namely,  that  in  certain  types  the  characters 
borne  by  the  female  and  the  male  germ-cells  of  the  same  plant 
are  demonstrably  different.     There   can  be  little  doubt   that 
further  research  will  reveal  cognate  phenomena  in  many  unsus- 
pected places.     The  first  example  in  which  such  a  state  of  things 
was  proved  to  exist  is  that  of  the  Stocks  investigated  by  Miss 
Saunders.2     By  a  long  course  of  analysis  she  succeeded  in  estab- 
lishing in  1908  the  fact  that  if  a  plant  of  Matthiola  is  of  that 
eversporting  kind  which  gives  a  large  proportion  of  double- 
flowered   plants  among  its  offspring   (produced   by  self-fertili- 
sation), then  the  egg-cells  of  such  a  plant  are  mixed  in  type,  but 
the  pollen  of  the  same  plant  is  homogeneous.     Some  of  the  egg- 
cells  have  in  them  the  two  factors  for  singleness,  but  some  of 
them  are  short  of  one  or  both  of  these  factors.     The  pollen- 
grains,  however,  are  all  recessives,  containing  neither  of  these 
factors.     The  egg-cells,  in  other  words,  are  mixed,  "singles"  and 
"doubles,"  while  the  pollen-grains  are  all  "doubles."     The  same 
is  true  of  the  factor  differentiating  "white,"  or  colourless  plastids 
from  cream-coloured  plastids  in  Matthiola,  the  egg-cells  being 
mixed  "whites"  and  "creams,"  while  the  pollen-grains  are  all 
"creams,"    viz:   recessives.      Later   in    the    same   year    (1908) 
de  Vries3  announced  a  remarkable  case  which  will  be  discussed 
in  detail  subsequently.     It  relates  to  certain  Oenotheras  hetero- 
zygous for  dwarf  ness,  in  which  (p.  113)  the  ovules  were  mixed, 
tails  and  dwarfs,  while  the  pollen  is  all  dwarf. 

Again  in  Petunia  Miss  Saunders's4  work  has  shown  that 
a  somewhat  similar  state  of  things  exists,  but  with  this  remark- 
able difference,  that  though  the  egg-cells  are  mixed,  singles  and 
doubles,  the  pollen-grains  are  all  singles,  viz:  dominants.  All 
the  Petunias  yet  examined  have  been  in  this  condition,  including 

2  Rep.  Evol.  Ctee.  R.  S.,  IV,  1908,  p.  38. 

3  Ber.  Deut.  Bot.  Ges.,  1908,  XXVI,  a,  p.  672. 

4  Jour.  Genetics,  i,  1910,  p.  57. 


THE  MUTATION  THEORY  105 

some  which  in  botanic  gardens  pass  for  original  species.  Whether 
actual  wild  plants  from  their  native  habitats  are  in  the  same 
state,  is  not  yet  known,  but  it  is  by  no  means  improbable.  The 
case  may  be  compared  with  that  of  the  moth  Abraxas  grossu- 
lariata  studied  by  Doncaster  and  Raynor,  in  which  the  females 
are  all  heterozygous,  or  we  may  almost  say  "hybrids"  of  grossu- 
lariata  and  the  variety  lacticolor.  Similarly  we  may  say  that  at 
least  garden  Petunias  are  heterozygous  in  respect  of  singleness. 
The  proof  of  this  is  of  course  that  when  fertilised  with  the  pollen 
of  doubles  they  throw  a  mixture  of  doubles  and  singles.  The 
statements  which  de  Vries  has  published  regarding  the  behaviour 
of  several  of  the  Oenotheras  go  far  to  show  that  they  must  have 
a  somewhat  similar  organisation.  On  the  present  evidence  it  is 
still  quite  impossible  to  construct  a  coherent  scheme  which  will 
represent  all  the  phenomena  in  their  interrelations,  and  among 
the  facts  are  several  which,  as  will  appear,  seem  mutually  incom- 
patible. The  first  indication  that  the  Oenotheras  may  have 
either  mixed  ovules  or  mixed  pollen  appears  in  the  fact  that 
Lamarckiana  and  several  of  its  "mutants"  used  as  males,  with 
several  other  forms  as  females,  give  a  mixed  offspring.  For 
example,  de  Vries  (1907)  found  that 

biennis  9   X  Lamarckiana  c? 

biennis  cruciata  9   X  Lamarckiana  cf 

muricata  9   X  Lamarckiana  cf 

biennis  9   X  rubrinervis  d71 

biennis  cruciata  9   X  rubrinervis  c? 

all  give  a  mixture  of  two  distinct  types  which  he  names  laeta 
and  velutina,  consisting  of  about  equal  numbers  of  each.  On 
account  of  the  fact  that  the  two  forms  are  produced  in  association 
de  Vries  has  called  these  forms  "twin  hybrids,"  a  designation 
which  is  not  fortunate,  seeing  that  it  is  impossible  to  imagine 
that  any  kind  of  twinning  is  concerned  in  their  production.  The 
distinction  between  these  two  seems  to  be  considerable,  laeta 
having  leaves  broader,  bright  green  in  colour,  and  flat,  with 
pollen  scanty,  while  vehitina  has  leaves  narrower,  grayish  green, 
more  hairy,  and  furrow-shaped,  with  pollen  abundant. 

We  next  meet  the  remarkable  fact  that  these  two  forms, 


io6  PROBLEMS  OF  GENETICS 

laeta  and  velutina  breed  true  to  their  respective  types,  and  do  not 
reproduce  the  parent-types  among  their  offspring  resulting  from 
self-fertilisation.  This  statement  must  be  qualified  in  two 
respects.  When  muricata  cf  is  fertilised  by  brevistylis  the  forms 
laeta  and  velutina  are  produced,  but  each  of  them  subsequently 
throws  the  short-styled  form  as  a  recessive  (de  Vries,  1907, 
p.  406).  It  may  be  remembered  that  de  Vries's  previous  publi- 
cations had  already  shown  that  the  short  style  of  brevistylis, 
one  of  the  Lamarckiana  "mutants,"  behaves  as  a  recessive 
habitually  (Mutationstheorie,  II,  p.  178,  etc.). 

Also  when  nanella,  the  dwarf  "mutant"  of  Lamarckiana  is 
used  as  male  on  muricata  as  female,  laeta  and  velutina  are  pro- 
duced, but  one  only  of  these,  namely,  velutina,  subsequently 
throws  dwarfs  on  self -fertilisation.  The  dwarfs  thus  thrown  are 
said  to  form  about  50  per  cent,  of  the  families  in  which  they 
occur  (de  Vries,  1908,  p.  668).  The  fact  that  the  two  forms, 
laeta  and  velutina,  are  produced  by  many  matings  in  which 
Lamarckiana  and  its  mutant  rubrinervis  are  used  as  males  is 
confirmed  abundantly  by  Honing,  who  has  carried  out  extensive 
researches  on  the  subject.  After  carefully  reading  his  paper, 
I  have  failed  to  understand  the  main  purport  of  the  argument 
respecting  the  "double  nature"  of  Lamarckiana  which  he  founds 
on  these  results,  but  I  gather  that  in  some  way  laeta  is  shown  to 
partake  especially  of  the  nature  of  Lamarckiana,  while  velutina 
is  a  form  of  rubrinervis.  The  paper  contains  many  records  which 
will  be  of  value  in  subsequent  analysis  of  these  forms. 

Before  considering  the  possible  meaning  of  these  facts  we 
must  have  in  our  minds  the  next  and  most  novel  of  the  recent 
extensions  of  knowledge  as  to  the  genetic  properties  of  the 
Oenotheras.  In  the  previous  statement  we  have  been  concerned 
with  the  results  of  using  either  Lamarckiana  itself  or  one  of  its 
"mutants"  rubrinervis,  brevistylis,  or  nanella  as  male,  on  one  of 
the  species  biennis  or  muricata.  The  new  experiments  relate 
to  crosses  between  the  two  species  biennis  and  muricata 
themselves. 

De  Vries  found : 

I.  That  the  reciprocal  hybrids  from  these  two  species  differed, 


THE  MUTATION  THEORY  107 

biennisXmuricata  producing  one  type  of  Fi  and  muricataX 
biennis  producing  another.  Each  Fi  resembled  the  father  more 
than  the  mother. 

2.  That  each  of  the  hybrids  so  produced  breeds  true  on  self- 
fertilisation. 

3.  That  if  we  speak  of  the  hybrid  from  biennisXmuricata 
as  BM  and  of  the  reciprocal  as  MB,  then 

BMXMB 

gives  exclusively  offspring  of  biennis  type  but  that 

MB  XBM 

gives  exclusively  offspring  of  muricata  type.  Evidently,  apart 
from  all  controversy  as  to  the  significance  of  the  "mutants" 
of  Lamarckiana,  we  have  here  a  series  of  observations  of  the  first 
importance. 

The  fact  that  reciprocal  crossings  give  constantly  distinct 
results  must  be  taken  to  indicate  that  the  male  and  female  sides 
of  one,  if  not  of  both,  of  the  parents  are  different  in  respect  of 
characters  which  they  bear.  This  is  de  Vries's  view,  and  he 
concludes  rightly,  I  think,  that  the  evidence  from  all  the  experi- 
ments shows  that  both  biennis  and  muricata  are  in  this  condition, 
having  one  set  of  characters  represented  in  their  pollen-grains 
and  another  in  their  ovules.  The  plants  breed  true,  but  their 
somatic  structures  are  compounded  of  the  two  sets  of  elements 
which  pass  into  them  from  their  maternal  and  paternal  sides 
respectively.  This  possibility  that  species  may  exist  of  which 
the  males  really  belong  to  one  form  and  the  females  to  another, 
is  one  which  it  was  evident  from  the  first  announcement  of  the 
discovery  of  Mendelian  segregation  might  be  found  realised  in 
nature.6 

Oe.  biennis  and  muricata  were  crossed  reciprocally  with  each 
other  and  with  a  number  of  other  species,  and  the  behaviour  of 
each,  when  used  as  mother,  was  consistently  different  from  its 
behaviour  when  used  as  father.  De  Vries  is  evidently  justified 

5  In  Rep.  i  to  Evol.  Committee,  1902,  p.  132,  attention  was  called  to  this  pos- 
sibility, though  of  course  at  that  date  it  was  in  sexual  animals  alone  that  it  was 
supposed  to  exist.  It  had  not  occurred  to  me  that  even  a  hermaphrodite  plant 
might  be  hi  this  condition. 


io8  PROBLEMS  OF  GENETICS 

by  the  results  of  this  series  of  experiments  in  stating  that  the 
"Bild,"  as  he  terms  it,  or  composition  of  the  male  and  female 
sides  of  these  two  species,  biennis  and  muricata,  are  distinct. 
On  the  evidence  before  us  it  is  not,  however,  possible  to  form  a 
perfectly  clear  idea  of  each,  and  until  details  are  published,  a 
reader  without  personal  knowledge  of  the  material  cannot  do 
more  than  follow  the  general  course  of  the  argument.  For  fuller 
comprehension  a  proper  analysis  of  the  characters  with  a  clear 
statement  of  how  they  are  distributed  among  the  several  types 
and  crosses  is  absolutely  necessary.  According  to  de  Vries  the 
female  of  biennis  possesses  a  group  of  characters  which  he  defines 
as  "conica"  in  allusion  to  the  shape  of  the  flower-buds.  Besides 
the  conical  buds,  this  group  of  features  includes  imperfect 
development  of  wood,  rendering  the  plant  very  liable  to  attacks 
of  Botrytis,  and  comparatively  narrow  leaves. 

The  female  of  muricata  carries  a  group  of  features  which  he 
calls  "frigida"  and,  though  this  is  not  quite  explicitly  stated  in  a 
definition  of  that  type,  it  is  to  be  inferred6  that  its  characteristics 
are  regarded  as  greater  height,  strong  development  of  wood  with 
comparative  resistance  to  Botrytis,  and  broad  leaves. 

The  characters  borne  by  the  male  parts  of  the  two  species 
are  in  general  those  by  which  they  are  outwardly  distinguished. 
For  example,  the  leaves  of  Oe.  biennis  are  comparatively  broad 
and  are  bright  green,  while  those  of  muricata  are  much  narrower 
and  of  a  glaucous  green,  and  I  understand  that  de  Vries  regards 
these  properties  as  contributed  by  the  male  side  in  each  case  and 
to  be  carried  by  the  male  cells  of  each  species.  The  suggestion 
as  regards  biennis  and  muricata  comes  near  the  conception  often 
expressed  by  naturalists  in  former  times  (e.  g.,  Linnaeus)  and 
not  rarely  entertained  by  breeders  at  the  present  day,  that  the 
internal  structure  is  contributed  by  the  mother  and  the  external 
by  the  father. 

On  the  other  hand,  the  offspring  of  each  species  when  used 
as  mother  is  regarded  as  possessing  in  the  main  the  features  of 
the  maternal  "Bild,"  but  the  matter  is  naturally  complicated 
by  the  introduction  of  features  from  the  father's  side,  and  it  is 

6  From  the  description  of  the  offspring  of  muricata  used  as  mother. 


THE  MUTATION  THEORY  109 

here  especially  that  the  account  provided  is  at  present  unsatis- 
factory and  inconclusive.  There  seems,  however,  to  be  no  serious 
doubt  that  biennis  and  muricata  each  in  their  outward  appearance 
exhibit  on  the  whole  the  features  which  their  pollens  respectively 
carry,  and  that  the  features  borne  by  their  ovules  are  in  many 
respects  distinct. 

The  types  are  thus  "hybrids"  which  breed  true.  The  results 
of  intercrossing  them  each  way  are  again  " hybrids"  which  breed 
true.  It  will  be  remembered  that  on  former  occasions  de  Vries 
has  formulated  a  general  rule  that  species-hybrids  breed  true, 
but  that  the  cross-breds  raised  by  interbreeding  varieties  do  not. 
One  of  these  very  cases  was  quoted7  as  an  illustration  of  this 
principle,  viz:  muricata Xbiennis.  The  grounds  for  this  general 
statement  have  always  appeared  to  me  insufficient,  and  with  the 
further  knowledge  which  the  new  evidence  provides  we  are 
encouraged  to  hope  that  when  a  proper  factorial  analysis  of  the 
types  is  instituted  we  shall  find  that  the  phenomenon  of  a  con- 
stant hybrid  will  be  readily  brought  into  line  with  the  systems 
of  descent  already  worked  out  for  such  cases  as  that  of  the  Stocks, 
and  others  already  mentioned. 

In  further  discussion  of  these  facts  de  Vries  makes  a  suggestion 
which  seems  to  me  improbable.  Since  the  egg-cells  of  muricata, 
for  instance,  bear  a  certain  group  of  features  which  are  missing 
on  the  male  side,  and  conversely  the  pollen  bears  features  absent 
from  the  female  side,  he  is  inclined  to  regard  the  bad  pollen  grains 
as  the  bearers  of  the  missing  elements  of  the  male  side  and  to 
infer  that  there  must  similarly  be  defective  ovules  representing 
the  missing  elements  of  the  female  side.  No  consideration  is 
adduced  in  support  of  this  view  beyond  the  simple  fact  that  the 
characters  borne  by  male  and  female  are  dissimilar,  whereas 
it  would  be  more  in  accord  with  preconception  if  the  same  sets 
of  combinations  were  represented  in  each — as  in  a  normal 
Mendelian  case.  There  is  as  yet  no  instance  in  which  the  absence 
of  any  particular  class  of  gametes  has  been  shown  with  any 
plausibility  to  be  due  to  defective  viability,  though  there  are,  of 
course,  cases  in  which  certain  classes  of  zygotes  do  not  survive 

7  de  Vries,  Species  and  Varieties,  1905,  p.  259. 


no 


PROBLEMS  OF  GENETICS 


owing  to  defective  constitution  (e.  g.,  the  albinos  of  Antirrhinum 
studied  by  Baur,  and  the  homozygous  yellow  mice).  I  am 
rather  inclined  to  suppose  that  in  these  examples  of  hybrids 
breeding  true  we  shall  find  a  state  of  things  comparable  with 
that  to  which  we  formerly  applied  the  terms  " coupling"  and 
"repulsion."  In  these  cases  certain  of  the  possible  combinations 
of  factors  occur  in  the  gametic  series  with  special  frequency, 
being  in  excess,  while  the  gametes  representing  other  combina- 
tions are  comparatively  few.  In  a  recent  paper  on  these  cases 
Professor  Punnett  and  I  have  shown  that  these  curious  results 
vary  according  to  the  manner  in  which  the  factors  are  grouped 
in  the  parents.  If  A  and  B  are  two  factors  which  exhibit  these 
phenomena  we  find  that  the  gametic  series  of  the  double  heterozy- 
gote  differs  according  as  the  combination  is  made  by  crossing 
ABXab,  or  by  crossing  AbXaB.  In  a  normal  Mendelian  case 
the  FI  form,  AaBb,  produces  gametes  AB,  Ab,  aB,  ab,  in  equal 
numbers;  but  in  these  peculiar  cases  those  gametes  which  contain 

Gametic  series 


AB        Ab 

aB           ab 

u  gaiiicica 

in  series 

^yguic 
forme( 

Partial  repulsion 
from  zygote     . 
of  form 

3i 
IS 

(n-i)       I 
31            I 
IS            I 

2« 
64 
32 

4tt2 
4096 
IO24 

AbXaB 

7 

7            I 

16 

256 

3 

3          i 

8 

64 

i 

i 

4 

16 

!3           I 

3 

8 

64 

Partial  coupling 

7          I 

7 

16 

256 

from  zygote     ^ 

IS          i 

IS 

32 

1024 

of  form 

3i 

64 

4096 

ABXab 

63          I 

63 

128 

16384 

(n-i)      i 

i     (w-i) 

2n 

4n2 

Nature  of  zygotic  series 


AB 


Ab 


aB 


ab 


Partial  repulsion 
from  zygote     J 
of  form 

2W-|-I 
2049 
513 

nf  —  i 
1023 
2SS 

W—  I 

1023 
255 

AbXaB 

129 

63 

63 

(^ 

33 

IS 

IS 

9 

3 

3 

41 

7 

7 

9 

177 

IS 

IS 

49 

737 

225 

3009 

63 

63 

961 

12161 

127 

127 

3969 

—  i  2n—i  n2  — 2w  — 


THE  MUTATION  THEORY  in 

the  parental  combinations  are  in  excess.  This  excess  almost 
certainly  follows  the  system  indicated  by  the  accompanying 
table.  In  the  general  expressions  n  is  half  the  number  of  gametes 
required  to  express  the  whole  system.  Now  if  we  imagine  that 
sex-factors  are  involved  with  the  others  concerned  in  such  a  re- 
lationship as  this  we  have  a  system  of  distribution  approximating 
to  that  found  in  biennis  and  muricata.  The  difference  in  re- 
ciprocals is  represented  in  a  not  improbable  way.  It  cannot  yet 
be  said  that  the  rarer  terms  in  the  series  are  formed  at  all,  and 
perhaps  they  are  not.  As  we  pointed  out  in  our  discussion  of 
these  phenomena,  the  peculiar  distribution  of  factors  in  these 
cases  must  be  taken  to  mean  that  the  planes  of  division  at  some 
critical  stage  in  the  segregation  are  determined  with  reference 
to  the  parental  groups  of  factors,  or  in  other  words,  that  the 
whole  system  has  a  polarity,  and  that  the  distribution  of  factors 
with  reference  to  this  polarity  differs  according  to  the  grouping 
of  factors  in  the  gametes  which  united  in  fertilization  to  produce 
the  plant.  Subsequent  proliferation  of  cells  representing  certain 
combinations  would  then  lead  to  excess  of  the  gametes  bearing 
them.  It  is  on  similar  lines  that  I  anticipate  we  shall  hereafter 
find  the  interpretation  of  the  curious  facts  discovered  by  de  Vries, 
though  it  is  evident  that  a  long  course  of  experiment  and  analysis 
must  be  carried  through  before  any  certainty  is  reached.  The 
work  must  be  begun  by  a  careful  study  of  the  descent  of  some 
single  factor,  for  example,  that  causing  the  broader  leaf  of 
biennis,  and  we  may  hope  that  the  study  of  Oenothera  by  proper 
analytical  methods  will  no  longer  be  deferred. 

We  have  now  to  return  to  the  relations  of  laeta  and  velutina. 
These  two  forms,  it  will  be  remembered  are  frequently  produced 
when  Lamarckiana  or  one  of  its  derivatives  is  used  as  male, 
and  the  most  unexpected  feature  in  their  behaviour  is  that  both 
breed  true  as  regards  their  essential  characteristics,  on  self-fertili- 
sation. If  one  only  bred  true  the  case  might,  in  view  of  the 
approximate  numerical  equality  of  the  two  types,  be  difficult 
to  interpret  on  ordinary  lines,  but  as  both  breed  true  it  must  be 
clear  that  some  quite  special  system  of  segregation  is  at  work. 
What  this  may  be  cannot  be  detected  on  the  evidence,  but  with 


H2  PROBLEMS  OF  GENETICS 

the  results  from  the  biennis-muricata  experiments  before  us, 
it  is  natural  to  suspect  that  we  may  here  again  have  to  recognise 
a  process  of  allocation  of  different  factors  to  the  male  and  female 
sides  in  laeta  and  velutina.  That  some  such  system  is  in  operation 
becomes  the  more  probable  from  the  new  fact  which  de  Vries 
states  in  describing  the  group  of  characters  which  he  calls  conica, 
namely  that  this  type  is  the  same  as  that  of  velutina. 

There  are  many  collateral  observations  recorded  both  by 
de  Vries  and  others  which  have  a  bearing  on  the  problems,  but 
they  do  not  yet  fall  into  a  coherent  scheme.  For  example,  we 
cannot  yet  represent  the  formation  of  laeta  and  velutina  from  the 
various  species  fertilised  by  Lamarckiana  cf .  That  this  is  not 
due  to  any  special  property  associated  with  the  pollen  of  La- 
marckiana is  shown  by  the  fact  that  a  species  called  Hookeri 
gives  laeta  and  velutina  in  both  its  reciprocal  crosses  with  La- 
marckiana (de  Vries,  1909,  p.  3),  and  also  by  the  similar  fact  that 
Lamarckiana  9  fertilised  by  the  pollen  of  a  peculiar  race  of 
biennis  named  biennis  Chicago  throws  the  same  types.  Before 
these  very  complicated  phenomena  can  be  usefully  discussed 
particulars  must  be  provided  as  to  the  individuality  of  the  various 
plants  used.  This  criticism  applies  to  much  of  the  work  which 
de  Vries  has  lately  published,  for,  as  we  now  know  familiarly, 
plants  to  which  the  same  name  applies  can  be  quite  different  in 
genetic  composition. 

Attention  should  also  be  called  to  one  curiously  paradoxical 
series  of  results.  When  the  dwarf  "mutant"  of  Lamarckiana 
which  de  Vries  names  "nanella"  is  used  as  father  on  muricatay 
Fi  consists  of  laeta  and  velutina  in  approximately  equal  numbers. 
Both  forms  breed  true  to  their  special  characteristics,  but 
velutina  throws  dwarfs  of  its  own  type,  while  laeta  does  not 
throw  dwarfs.  Subsequent  investigation  of  the  properties  of 
these  types  has  led  to  some  remarkable  conclusions,  and  it  was 
in  a  study  of  these  plants  that  de  Vries  first  came  upon  the  phe- 
nomen  of  dissimilarity  between  the  factors  borne  by  the  male 
and  female  cells  of  the  same  plant,  a  condition  which  had  been 
recently  detected  in  the  Stocks  as  a  result  of  Miss  Saunders's 
investigations.  The  details  are  very  remarkable.  We  have 


THE  MUTATION  THEORY  113 

first  the  fact  that  muricata  9  X  dwarf  nanella  c?   gives  about 
50  per  cent,  laeta  and  about  50  per  cent,  of  velutina. 
As  regards  Velutina  it  was  shown  that: 

Tails,  Dwarfs, 

per  cent.  per  cent. 

I.        Velutina  selfed  gave                                                      38  62 

Velutina  9    X  dwarf  nanella  c?  gave                           39  61 

do.          X           do.               gave                          49  51 

do.          X  dwarf  c?  derived  from  velutina  gave  43  57 
3.       Dwarfs  X  velutina  cf  gave                                         —     all  dwarfs 

The  three  experiments  taken  together  prove,  as  de  Vries  says, 
that  the  ovules  of  velutina  are  mixed,  tails  and  dwarfs,  and  that 
the  pollen  is  all  dwarf.  The  condition  is  almost  the  same  as 
that  of  the  Stocks.  It  may  be  noted  also  that  in  the  Stocks  the 
egg-cells  of  the  "double"  type  are  in  excess,  being  approximately 
9  to  7  of  the  "single"  type,  but  de  Vries  regards  the  two  types 
in  velutina  as  probably  equal  in  number.  The  figures  (169  : 231) 
rather  suggest  some  excess  of  the  recessives,  perhaps  9:7,  and 
the  point  would  be  worth  a  further  investigation. 

As  regards  laeta,  by  self -fertilisation  no  dwarfs  were  produced, 
but  in  all  other  respects  it  behaved  almost  exactly  like  velutina. 
The  ovules  are  evidently  mixed  tails  and  dwarfs,  and  whether 
fertilised  by  dwarfs  or  by  the  pollen  of  velutina,  which  is  already 
proved  to  be  all  dwarf,  the  result  was  a  steady  50  per  cent,  of 
tails  and  50  per  cent,  of  dwarfs.  The  pollen  of  laeta  used  on 
dwarfs  gives  nothing  but  dwarfs,  and  in  three  series  of  such  ex- 
periments 226  dwarfs  were  produced. 

We  are  thus  faced  with  this  difficulty.  Since  the  egg-cells  of 
laeta  are  evidently  mixed,  tails  and  dwarfs,  and  the  pollen  used 
on  dwarfs  gives  all  dwarfs,  why  does  not  self-fertilisation  give 
a  mixed  result,  tails  and  dwarfs,  instead  of  all  tails?  De  Vries 
regards  the  result  of  self-fertilisation  as  showing  the  real  nature 
of  the  pollen,  and  declares  it  to  be  all  tails,  while  he  represents 
the  behaviour  of  the  same  pollen  used  on  dwarfs  by  stating  that 
in  these  combinations  the  dwarf  character  dominates.  This 
does  not  seem  to  me  a  natural  interpretation.  I  should  regard 
the  pollen  of  laeta  as  identical  with  that  of  velutina,  namely  dwarf, 
and  I  suspect  the  difficulty  is  really  created  by  the  behaviour  of 
laeta  on  self-fertilisation.  Until  a  proper  analysis  is  made  in 


114  PROBLEMS  OF  GENETICS 

which  the  identity  of  the  different  individuals  used  is  recorded, 
no  further  discussion  is  possible.8 

Other  results  of  a  complicated  kind  involving  production  of 
laeta  and  velutina  together  with  a  third  form  have  been  published 
by  de  Vries  in  his  paper  on  "Triple  Hybrids."  To  these  also 
the  same  criticism  applies.  Some  of  the  observations  seem  cap- 
able of  simple  factorial  representation  and  others  are  conflicting. 

Taking  the  work  on  Oenothera  as  a  whole  we  see  in  it  con- 
tinually glimpses  of  order  which  further  on  are  still  blocked  by 
difficulties  and  apparent  inconsistencies.  Through  such  a  stage 
all  the  successful  researches  in  complicated  factorial  analysis 
have  passed  and  I  see  no  reason  for  supposing  that  with  the 
application  of  more  stringent  methods  this  more  difficult  set  of 
problems  will  be  found  incapable  of  similar  solutions.  To 
return  to  the  original  question  whether  in  Oenothera  we  can  claim 
to  see  a  special  contemporaneous  output  of  new  species  in  actual 
process  of  creation,  it  will  be  obvious  that  while  the  interrelation 
of  the  several  types  is  still  so  little  understood,  such  a  claim  has 
no  adequate  support.  It  is  true  that  many  of  the  "mutants" 
of  Lamarckiana  can  well  pass  for  species,  but  this  is  equally  true 
of  many  new  combinations  of  pre-existing  factors  as  we  have 
seen  in  Primula  Sinensis  and  other  cases.  Still  less  can  it  be 
admitted  that  these  facts  of  uncertain  import  supply  a  justi- 
fication for  the  conception  which  has  played  a  prominent  part 
in  the  scheme  of  the  Mutationstheorie,  namely  that  there  are 
special  periods  of  Mutation,  when  the  parent-species  has  peculiar 
genetic  properties.  To  conclude:  The  impression  which  the 
evidence  leaves  most  definitely  on  the  mind  is  that  further  dis- 
cussion of  the  bearing  which  the  Oenotheras  may  have  on  the 
problem  of  evolution  should  be  postponed  until  we  have  before 
us  the  results  of  a  searching  analysis  applied  to  a  limited  part  of 
the  field.  In  such  an  analysis  it  is  to  be  especially  remembered 
that  we  have  now  a  new  clue  in  the  well-ascertained  fact  that  the 
genetic  composition  of  the  male  and  female  germ-cells  of  the 

8  Zeijlstra  in  a  recent  paper  announces  that  many  nanella  plants  are  the  subject 
of  a  bacterial  disease  to  which  he  attributes  their  dwarfness.  I  gather  that  this 
does  not  apply  to  all  nanella  plants  and  that  some  are  dwarfs  apart  from  disease. 
The  matter  may  no  doubt  be  further  complicated  from  this  cause. 


THE  MUTATION  THEORY  115 

same  individual  may  be  quite  different.  When  with  this  pos- 
sibility in  view  the  behaviour  of  the  types  is  re-examined  I 
anticipate  that  many  of  the  difficulties  will  be  removed. 

Outside  the  evidence  from  Oenothera,  which,  as  we  have  seen, 
is  still  ambiguous,  I  know  no  considerable  body  of  facts  favour- 
able to  that  special  view  of  Mutation  which  cfe  Vries  has  pro- 
mulgated. Of  variation,  or  if  we  will,  Mutation,  in  respect  of 
some  one  character,  or  resulting  from  recombination,  there  is 
proof  in  abundance ;  but  of  that  simultaneous  variation  in  several 
independent  respects  to  which  de  Vries  especially  attributes  the 
origin  of  new  specific  types  I  know  only  casual  records  which 
have  yet  to  undergo  the  process  of  criticism. 


Besides  de  Vries's  " Mutationstheorie"  and  "Species  and  Vari- 
eties" the  chief  publications  relating  to  the  subject  of  the  be- 
haviour of  Oenothera  are  the  following:  (Many  other  papers 
relating  especially  to  the  cytology  of  the  forms  have  appeared.) 

Davis,  B.  M.     Genetical  Studies  on  Oenothera,  I.     Amer.  Nat.,  XLIV,  1910,  p.  108. 

Genetical  Studies  on  Oenothera,  II.     Ibid.,  XLV,  1911,  p.  193. 
Gates,  R.  R.     An  Analytical  Key  to  some  of  the  Segregates  of  Oenothera.     Twen- 
tieth Annual  Report  of  the  Missouri  Botanical  Garden,  1909. 
Studies  on  the  Variability  and  Heritability  of  Pigmentation  in  Oenothera. 

Ztsch.f.  Abstammungslehre,  1911,  IV,  p.  337. 

Honing,  J.  A.  Die  Doppelnatur  der  Oenothera  Lamarckiana.  Ztsch.  f.  Abstam- 
mungslehre, 1911,  IV,  p.  227. 

Macdougal,  D.  T.  (with  A.  M.  Vail,  G.  H.  Shull,  and  J.  K.  Small).  Mutants  and 
Hybrids  of  the  Oenotheras.  Carnegie  Institution's  Publication,  No.  24, 
1905. 

Macdougal,  D.  T.,  Vail,  A.  M.,  Shull,  J.  H.     Mutations,  Variations  and  Relation- 
ships of  the  Oenotheras.     Carnegie  Institution's  Publication,  No.  81,  1907. 
de  Vries,  H.     On  Atavistic  Variation  in  Oenothera  cruciata.    Bull.  Torrey  Club, 

1903.  Vol.  30,  p.  75- 

On  Twin  Hybrids,  Bot.  Gaz.,  Vol.  44,  1907,  p.  401. 
Ueber  die  Zwillingsbastarde  von  Oenothera  nanella.    Ber.  Deut.  Bot.  Ges., 

1908,  XXVI,  a,  p.  667. 

Bastarde  von  Oenothera  gigas.     Ibid.,  p.  754. 
On  Triple  Hybrids.     Bot.  Gaz.,  1909,  Vol.  47,  p.  I. 
Ueb.   doppeltreziproke   Bastarde   von   Oenothera   biennis  L.   und   Oenothera 

muricata  L.     Biol.  Cbltt.,  1911,  XXXI,  p.  97. 

Zeijlstra,  H.  H.  Oenothera  nanella  de  Vries,  eine  krankhafte  Pflanzenart.  Biol. 
Cbltt.,  1911,  XXXI,  p.  129. 


Ii6  PROBLEMS  OF  GENETICS 

NOTE. 

Since  this  chapter  was  written  two  contributions  of  special 
importance  have  been  made  to  the  study  of  the  Oenothera  prob- 
lems. The  first  is  that  of  Heribert-Nilsson.9  The  author  begins 
by  giving  a  critical  account  of  the  evidence  for  de  Vries's  inter- 
pretation of  the  nature  of  the  mutants.  In  general  this  criticism 
pursues  lines  similar  to  those  sketched  in  the  foregoing  chapter, 
concluding,  as  I  have  done,  that  the  chief  reason  why  factorial 
analysis  has  been  declared  to  be  inapplicable  to  the  Oenothera 
mutants  is  because  no  one  has  hitherto  set  about  this  analysis 
in  the  right  way.  He  has  also  himself  made  a  valuable  beginning 
of  such  an  analysis  and  gives  good  evidential  reasons  for  the  belief 
that  at  least  the  red  veining  depends  on  a  definite  factor  which 
also  influences  the  size  of  certain  parts  of  the  plant.  He  argues 
further  that  many  of  the  distinctions  between  the  mutants  are 
quantitative  in  nature.  With  great  plausibility  he  suggests  that 
the  system  of  cumulative  factors  which  Nilsson-Ehle  discovered 
in  the  case  of  wheat  (subsequently  traced  by  East  in  regard  to 
maize)  may  be  operating  also  in  these  Oenotheras.  According 
to  this  system  several  factors  having  similar  powers  may  coexist 
in  the  same  individual,  and  together  produce  a  cumulative  effect. 
Scope  would  thus  be  given  for  the  production  of  the  curious  and 
seemingly  irregular  numbers  so  often  recorded  in  the  "mutating" 
families. 

Another  remarkable  observation  relating  to  the  crosses  of 
muricata  and  biennis  has  been  published  by  Goldschmidt.10 
He  finds  that  in  the  formation  of  this  cross  the  female  pronucleus 
takes  no  part  in  the  development  of  the  zygotic  cell,  but  that 
when  the  male  pronucleus  enters,  the  female  pronucleus  is 
pushed  aside  and  degenerates.  As  de  Vries  observed,  the  recip- 
rocal hybrids  are  in  each  case  very  like  the  father  ("stark 
patroklin"),  a  consequence  which  finds  a  natural  explanation  in 
the  phenomenon  witnessed  by  Goldschmidt.  The  results  of 
the  subsequent  matings  can  also  be  readily  interpreted  on  the 
same  lines.  Indications  of  maternal  characters  are  nevertheless 

9  Zts.  f.  Abstamm.,  1912,  VIII. 

10  Arch.  f.  Zellforschung,  1912,  IX,  p.  331. 


THE  MUTATION  THEORY  117 

mentioned  by  de  Vries,  and  if  Goldschmidt's  account  of  the 
cytology  is  confirmed,  these  must  presumably  be  referred  to  the 
influence  of  the  maternal  cytoplasm.  Clearly  this  new  work 
opens  up  lines  of  exceptional  interest.  The  interpretation  I 
have  offered  above  must  probably  be  reconsidered.  The  dis- 
tinction between  the  male  and  female  cells  of  the  types  may  no 
doubt  be  ultimately  factorial,  but  it  is  difficult  to  regard  such  a 
distinction  as  created  by  a  differential  distribution  of  the  ordinary 
factors. 


CHAPTER  VI 

VARIATION   AND  LOCALITY 

In  all  discussions  of  the  modes  of  Evolution  the  phenomena  of 
Geographical  Distribution  have  been  admitted  to  be  of  para- 
mount importance.  First  came  the  broad  question,  were  the 
facts  of  distribution  consistent  with  the  Doctrine  of  Descent? 
I  suppose  all  naturalists  are  now  agreed  that  they  are  thus 
consistent,  and  that  though  some  very  curious  and  as  yet  in- 
explicable cases  remain  to  be  accounted  for,  the  distribution  of 
animal  and  plant  life  on  the  face  of  the  earth  is  much  what  we 
might  expect  as  a  result  of  a  process  of  descent  with  modification. 
Passing  from  this  general  admission  to  the  more  particular  ques- 
tion whether  the  facts  of  distribution  favour  one  special  con- 
ception of  the  mode  of  progress  of  evolution  rather  than  another, 
no  agreement  has  yet  been  reached.  One  outstanding  feature 
is  hardly  in  dispute,  namely  that  prolonged  isolation  is  generally 
followed  by  greater  or  less  change  in  the  population  isolated. 
Groups  of  individuals  which  from  various  causes  are  debarred 
from  free  intermixture  with  other  groups  almost  always  exhibit 
peculiarities,  but  on  the  other  hand,  cosmopolitan  types  which 
range  over  wide  areas  are  on  the  whole  uniform,  or  nearly  so 
throughout  their  distribution.  Examples  of  these  two  categories 
will  be  familiar  to  all  naturalists.  The  barriers  to  intercourse 
may  be  seas,  deserts,  prairies,  mountain-chains,  or  circumstances 
of  a  much  less  obvious  character  which  isolate  quite  as  effectually. 
The  local  unit  is  not  necessarily  an  island,  a  district,  or  an  area 
of  special  geological  formation,  but  may,  as  every  collector  knows, 
be  a  valley,  a  pond,  a  creek,  a  "bank"  in  the  sea,  a  clump  of 
trees,  a  group  of  rocks  in  a  bay,  or  a  particular  patch  of  ground 
on  a  mountain  side.  All  the  great  groups  provide  examples  of 
such  specially  isolated  forms.  The  botanist  knows  them  well; 
the  conchologist,  the  entomologist,  the  ornithologist  and  the 
student  of  marine  life  are  all  equally  aware  that  special  varieties 

118 


VARIATION  AND  LOCALITY  119 

or  special  species  come  from  special  places  and  from  nowhere 
else.  In  one  remarkable  case  the  season  of  appearance  plainly 
acts  as  the  isolating  barrier.  Tephrosia  bistortata  is  a  small 
Geometrid  moth  which  has  two  broods,  appearing  in  March 
and  July  respectively.  It  is  closely  allied  to  T.  crepuscularia 
which  emerges  in  May  and  June.  From  the  fact  that  occasional 
specimens  cannot  be  quite  certainly  referred  to  one  or  other  of 
the  two,  many  have  held  that  the  two  are  one  species.  Never- 
theless, in  general  they  present  distinctions  which  are  plain 
enough.  Some  localities  have  one  form  only,  but  in  several 
woods  they  co-exist.  Experiment  has  shown  that  the  two  can 
be  crossed,  and  that  the  cross-breds  can  breed  inter  se  and  with 
at  least  one  of  the  parent  stocks.1  Some  diminution  in  fertility 
was  observed,  but  perhaps  not  more  than  is  commonly  encountered 
when  wild  forms  are  bred  in  captivity.  In  such  a  case  it  can 
scarcely  be  doubted  that  the  distinctness  of  the  two  forms  in 
the  places  where  they  co-exist  is  maintained  by  the  seasonal 
isolation. 

Just  as  the  consequences  of  isolation  are  to  be  seen  in  the 
most  different  forms  of  life  so  may  they  also  affect  the  most  di- 
verse features  of  organisation,  such  as  size,  colour,  sculpture, 
shape,  or  number  of  parts.  In  the  Sloth  (Choloepus)  the  geo- 
graphical races  differ  in  the  number  of  cervical  vertebrae — or 
in  other  words,  in  the  distribution  of  vertebral  differentiation. 
The  geographical  races  of  Cistudo  differ  in  the  number  of  claws 
and  phalanges.2 

In  Shetland,  the  males  of  Hepialus  humuli  (the  Ghost  Moth) 
are  not  sharply  differentiated  in  colour  from  the  females,  as  they 
are  elsewhere,  but  in  varying  degrees  resemble  them.8  No  such 
males  are  found  in  other  localities,  and  even  in  the  other  Scottish 
islands  they  are  normal.  In  the  island  of  Waigiu  the  converse 
phenomenon  has  been  observed  in  Phalanger  maculatus.  Gen- 

1  For  the  evidence  see  Tutt,  J.  W.,  Trans.  Ent.  Soc.,  1898,  p.  17.     Compare 
the  remarkable  case  given  by  Gulick  (Evolution  Racial  and    Habitudinal,  p.  123) 
of  the  two  races  of  Cicada,  which  are  separated  by  reason  of  their  life-cycles,  one 
having  a  period  of  13,  the  other  17  years. 

2  For  references  see  Materials,  p.  396,  and  also  G.  Baur,  Amer.  Nat.,  1893, 
July,  p.  677. 

1  Jenner  Weir,  Entomologist,  1880,  XIII,  p.  251. 


120  PROBLEMS  OF  GENETICS 

erally  the  male  is  spotted  with  white,  and  the  female  is  without 
spots,  but  in  Waigiu  the  females  are  spotted  like  the  males.4 

The  following  striking  illustration  was  pointed  out  to  me  by 
Dr.  W.  D.  Miller.  Euphonia  elegantissima  as  it  occurs  in  Mexico 
and  Central  America  has  the  two  sexes  very  distinct  from  each 
other.  The  male  has  the  lower  parts  orange  and  the  upper 
parts  a  dark  indigo  blue,  with  a  bright  turquoise-blue  head  and 
neck.  The  female,  except  for  the  head,  is  of  a  bright  olive  green. 
A  form  in  which  the  sexes  are  similarly  differentiated  exists  in 
Porto  Rico  and  is  known  as  E.  Sclateri.  But  in  many  of  the 
other  West  Indian  islands  the  representative  "species"  (E. 
flavifrons)  has  the  two  sexes  closely  resembling  the  female  of 
E.  elegantissima.  This  form  is  found  in  Antigua,  Barbados, 
St.  Vincent,  and  Guadeloupe,  from  which  localities  the  British 
Museum  has  specimens.  All  three  so-called  species  are  very 
much  alike  otherwise. 

In  the  genus  Pyrrhulagra  (Loxigilla)  to  which  Mr.  Outram 
Bangs  called  my  attention,  several  distinct  and  alternative  pos- 
sibilities occur.  The  genus  has  many  local  species  occurring 
on  the  various  West  Indian  islands.  These  species  are  char- 
acterized by  differences  in  size,  colour,  and  the  shape  of  the  bill. 
The  colours  have  a  narrow  range,  being  black  or  greyish,  with 
or  without  chestnut  marks  about  the  head  and  throat.  In 
most  of  the  islands  the  males  are  in  general  colour  a  full  black, 
and  the  females  are  distinctly  grey.  They  are  thus  found  in 
San  Domingo,  Jamaica,  Bahama,  and  most  of  the  Lesser  Antilles. 
In  Porto  Rico  we  meet  the  peculiarity  that  the  hens  are  almost 
as  black  as  the  males  (Ridgway  describes  the  black  of  the  hens 
as  slightly  less  intense).  This  form  is  called  portoricensis. 
A  larger  type,  known  as  grandis,  similarly  coloured,  inhabits 
St.  Kitt's.  Then,  on  the  contrary,  in  Barbados,  both  sexes  are 
a  dull  blackish  grey,  like  the  hens  of  the  Lesser  Antilles  in  general. 

The  local  species  of  Agelaius  show  similarly  capricious  dis- 
tinctions. A.  phoeniceus  is  a  widely  spread  species,  found  over 
a  great  part  of  North  America.  The  male  is  black  with  red-orange 

4  Jentink,  Notes  Leyden  Mus.,  1885,  VII,  p.  in.  Specimens  illustrating  this 
peculiarity  are  in  the  British  Museum. 


VARIATION  AND  LOCALITY  121 

bars  on  the  wings,  but  the  female  is  somewhat  thrush-like  in 
colour.  In  the  island  of  Porto  Rico  there  is  a  form  called  xan- 
thomus,  in  which  both  sexes  are  like  the  males  of  the  mainland. 
A  similar  species  called  humeralis,  also  with  both  sexes  male-like, 
lives  in  Cuba.  The  island  of  Cuba,  curiously  enough,  has  also  a 
distinct  species  named  assimilis,  in  which  the  female  is  a  dull 
black  all  over,  though  the  male  is  like  the  mainland  type. 

So  also  may  local  races  differ  in  respect  of  variability.  Ar- 
gynnis  paphia,  the  Silver  Washed  Fritillary,  through  a  great 
part  of  its  distribution  has  only  one  female  form.  In  the  English 
New  Forest  a  second  female  form,  valesina,  co-exists  with  the 
ordinary  paphia  female.  But  in  the  southern  valleys  of  the  Alps 
the  valesina  female  is  much  the  commoner  of  the  two,  and  indeed 
in  some  localities  where  the  species  is  abundant,  I  have  seen  no 
paphia  females  in  many  days  collecting. 

The  beetle  Gonioctena  variabilis  furnishes  an  illustration  of  a 
comparable  phenomenon  affecting  the  male  sex.  In  1894  an(i 
1895  I  studied  the  curious  colour  variations  of  this  species  es- 
pecially in  the  neighbourhood  of  Granada,  and  Mr.  Doncaster 
ten  years  later  repeated  the  observations  on  the  same  ground, 
and  also  collected  the  insect  in  other  places  in  the  south  of  Spain. 
The  distinctions  are  not  easy  to  give  in  words  and  the  reader  is 
referred  to  the  colour  plate  accompanying  my  paper.5  The 
essential  fact  is  that  the  males  commonly  have  the  elytra  red 
with  black  spots  and  the  females  for  the  most  part  have  greenish 
grey  elytra  with  black  stripes.  In  some  localities  a  large  minority 
of  males  closely  resemble  the  female  type,  being  identical  in 
colour  and  then  only  distinguishable  by  structural  differences. 
In  two  Granada  localities  I  found  the  proportion  of  such  males 
quite  different.  In  the  Darro  valley  about  38  per  cent,  (in 

5  Proc.  ZooL  Soc.,  1895,  p.  850.  Plate.  Many  points  beyond  that  mentioned 
above  are  involved  in  this  remarkable  case.  For  example,  not  only  are  there  males 
like  females,  but  a  small  proportion  of  females  resemble  the  ordinary  male  type. 
The  stripes  are  not  merely  the  spots  produced,  for  they  occupy  different  anatomical 
positions.  The  spots  almost  always  go  with  a  black  ventral  surface,  but  the  striped 
forms  nearly  always  have  that  region  testaceous.  Spartium  retama,  the  food-plant, 
will  not  grow  in  England,  but  if  it  could  be  naturalised  in  America  the  whole  problem 
might  be  investigated  there  and  results  of  exceptional  interest  would  almost  cer- 
tainly be  attained. 


122  PROBLEMS  OF  GENETICS 

718)  were  of  this  feminine  type,  but  on  the  hills  some  300  feet 
above  only  19  per  cent,  (in  3,230)  were  like  the  females.  At 
Castillejo,  not  far  from  Toledo  I  found  no  such  male  in  75  speci- 
mens. 

Mr.  Doncaster  collected  from  several  localities,  especially 
from  two  areas  near  Malaga,  about  5  miles  apart.  In  one  of 
these  the  female-like  males  were,  as  usual,  in  a  minority,  but 
in  the  other  these  were  actually  in  great  excess,  amounting  to 
about  8 1  per  cent,  in  the  173  taken.  Doncaster  found  a  doubtful 
indication  that  the  composition  of  the  population  varies  with 
the  season,  which  is  quite  possible,  but  it  is  most  interesting 
to  note  that  in  my  chief  locality  after  the  lapse  of  ten  years  he 
found  the  proportions  very  much  the  same  as  I  had  done  at  the 
same  season,  for  where  I  had  19  per  cent,  of  the  female-like  males 
his  collecting  gave  16  per  cent.  In  other  respects  also,  his  sta- 
tistics corresponded  very  closely  with  mine.6 

The  various  forms  of  Heliconius  erato  are  well  known  to  en- 
tomologists. They  are  strikingly  distinguished  by  the  colours 
of  the  strong  comb-like  marking  on  the  hind  wing,  which  may  be 
red,  yellow,  green  or  blue.  In  various  parts  of  the  distribution 
in  South  America  sometimes  two  and  sometimes  three  of  these 
distinct  types  co-exist.7 

The  distribution  of  the  varieties  of  Noctua  castanea  typifies 
a  large  range  of  cases.  The  form  which  is  reckoned  the  normal 
of  the  species  has  red  fore-wings.  It  is  practically  restricted  to 
Great  Britain  and  Germany,  according  to  Tutt.  The  other 
common  form,  neglecta,  has  grey  fore-wings,  and  in  this  pattern 
it  ranges  through  West  Central  Europe  from  North  Italy  to 
Germany.  In  the  British  Isles  it  extends  up  to  Orkney.  In 
Britain  this  grey  form  is  by  far  the  commoner,  occurring  where- 

6  Doncaster,  L.,  Proc.  Zool.  Soc.,  1905,  II,  p.  528. 

7 1  am  not  aware  that  the  details  of  this  striking  case  have  ever  been  worked 
out.  It  should  be  noted  that  the  green  and  blue  forms  are  not  due  to  simple  modi- 
fication of  the  red  pigment;  for  these  colours,  due  to  interference,  fork  over  the 
area  occupied  by  the  red  lines.  The  distinctions  between  these  forms  cannot 
therefore  be  simply  chemical,  as  we  may  suppose  them  to  be,  for  instance,  in  the 
case  of  many  red  and  yellow  forms,  and  the  genetic  relationships  of  the  Hehconid 
varieties  would  raise  many  novel  problems  and  be  well  worth  studying  experi- 
mentally. 


VARIATION  AND  LOCALITY  123 

ever  the  species  is  found.  The  red  form  is  much  scarcer  in 
England,  and  does  not  occur  at  all  in  many  localities  where  the 
grey  form  is  common.  Mr.  Woodforde,  from  whom  this  account 
is  taken,8  states  that  in  August,  1899,  he  saw  considerably  over  a 
hundred  of  the  grey  in  the  New  Forest  at  sugar,  but  only  two 
red  ones.  In  Staffordshire  however  the  red  is  proportionately 
more  numerous  and  he  estimates  them  as  40  per  cent,  of  the 
population.  Lastly  a  form  has  been  taken  in  Staffordshire  as  a 
rarity  in  which  the  red  is  replaced  by  yellow,  and  this  has  hitherto 
been  seen  nowhere  else.  It  is  beyond  our  immediate  purposes 
to  discuss  the  genetic  relationships  of  such  forms,  but  the  details 
of  this  case  are  interesting  as  making  fairly  clear  the  fact  that 
the  distinctions  between  castanea  and  neglecta  are  due  to  com- 
binations of  the  presence  of  and  absence  of  two  pairs  of  factors, 
of  which  one  produces  a  red  pigment  in  the  ground  colour  of  the 
forewing  and  the  other  irrorates  the  same  region  with  black 
scales.  Mr.  Woodforde  states  that  all  intermediates  exist, 
and  that  in  Staffordshire  the  greys  always  have  a  pinkish  tinge. 
The  yellow  is  doubtless  another  recessive  to  the  red. 

Species  which  are  uniform  in  some  localities  may  be  poly- 
morphic in  others.  Such  a  phenomenon  is  well  exemplified  by 
the  orchid  Acer  as  hircina.  Of  this  species  distinct  varieties  had 
previously  been  known  in  Germany,  but  Galle 9  has  lately  given  a 
detailed  account  of  a  number  of  most  diverse  forms  found  growing 
in  a  district  of  Eastern  France.  Without  reference  to  his  plates 
it  is  impossible  to  give  any  adequate  conception  of  the  profusion 
of  types  which  the  flowers  of  the  species  there  assume.  In  some 
the  lip  is  elongated  to  many  times  its  usual  length,  twisting 
and  dividing  in  a  fashion  suggesting  some  of  the  strangest  of  the 
Tropical  Orchids.  In  others  the  labellum  and  the  lateral  petals 
are  all  comparatively  short  and  wide  (Fig.  13).  Intermediates, 
combining  these  qualities  in  various  degrees,  were  abundant,  and 
the  condition  of  the  species,  which  was  the  only  representative  of 
the  genus  in  the  locality,  recalls  the  extreme  polymorphism  of 
many  of  the  Noctuid  Moths. 

8  Woodeforde,  F.  C.,  Trans.  North  Staffordshire  Field  Club,  XXXV,  1901,  Plate. 

9  E.  Galle,  Compte  Rendus  du  Congres  Internal,  de  Bot.  a  I'Expos.  Univ.,  1900, 
p.  112. 


124 


PROBLEMS  OF  GENETICS 


FIG.  13.    Various  forms  of  Acer  as  hircina.     (After  Galle.)     This  figure  only  shows 
a  few  of  the  more  striking  forms  illustrated  in  Galle's  plates. 


VARIATION  AND  LOCALITY  125 

Somewhat  comparable  variability  has  been  seen  in  another 
Orchid  genus  Ophrys.  In  Great  Britain  the  species  apifera, 
aranifera  and  muscifera  though  variable  are  fairly  distinct,  but 
Moggridge  has  published  two  series  of  plates10  showing  a  very 
different  state  of  things  as  regards  the  Ophrys  population  of  the 
Riviera.  Here  the  outward  diversity  is  such  that  the  ordinary 
specific  names  cannot  be  applied  with  any  confidence  and  the 
limits  of  the  species  are  quite  uncertain.  It  may  well  be  supposed 
that  these  Riviera  plants  are  interbreeding,  and  indeed  we  may 
safely  assume  that  they  are.  It  is,  however,  to  be  remembered 
that  Darwin  showed  apifera  in  this  country  to  be  habitually  self- 
fertilised,  so  that  the  different  behaviour  on  the  Riviera  may 
itself  constitute  a  local  peculiarity.  Moreover  it  is  to  be  gathered 
from  Moggridge's  account  that  in  the  districts  which  he  examined 
the  condition  was  not  to  be  described  by  the  statement  that  our 
three  types  were  there  co-existing  and  hybridising,  but  rather 
we  should  say  that  the  population  was  polymorphic,  containing 
these  three  types  amongst  others.  Conchologists  are  aware 
that  on  the  Dogger  Bank  Modiola  attains  a  size  unparalleled 
elsewhere.  The  same  is  true  of  the  sponges  Grantia  compressa 
and  Grantia  ciliata  in  the  estuary  of  the  Orwell.11  Conversely, 
as  we  know  so  well  in  the  case  of  Man,  dwarf  races  occur  in 
several  special  localities.  Such  examples  may  be  multiplied 
indefinitely. 

The  relation  of  local  forms  to  species  has  often  been  dis- 
cussed from  many  points  of  view,  but  I  know  no  treatment  of 
the  subject  clearer  or  more  comprehensive  than  an  excellent 
account  of  some  of  the  various  manifestations  of  local  dif- 
ferentiation as  they  appear  in  Helicidae  published  by  Coutagne12 
and  a  reader  interested  in  the  problem  which  they  raise  would 

10  Flora  of  Mentone,  1864-8,  Nova  Ada  Acad.  Goes.,  XXXV,  1869. 

11  I  owe  these  facts  to  Canon  A.  M.  Norman,  who   showed    me   illustrative 
specimens.     They  were  originally  described  by  Bower  bank  (Monogr.  Brit.  Spongi- 
adae,  vol.  II,  pp.  18  and  XX;   vol.  Ill,  Pis.  I  and  III).     A  specimen  of  G.  compressa 
measured  5  inches,  with  a  greatest  width  of  3  M  in.     G.  ciliata  was  found  measuring 
3  in.  long  and  M  in.  wide.     These  dimensions  are  many  times  those  of  normal 
specimens. 

12  Coutagne,  G.,  Recherches  sur  le  Polymorphisme  des  Mollusques  de  France 
Annales  Soc.  d'Agric.  Set.  et  Industr.  Lyon,  1895. 


126  PROBLEMS  OF  GENETICS 

do  well  to  make  himself  acquainted  with  the  original  from 
which  the  following  notes  are  taken.  He  speaks  for  example 
of  Helix  lapicida.  This  is  on  the  whole  a  constant  form  ranging 
up  to  the  altitude  of  1,300  m.,  common  all  over  France  except 
at  great  heights  and  in  the  Olive  regions  where  it  is  restricted 
to  moist  places.  Though  subjected  to  such  diverse  conditions 
it  shows  only  trivial  variations  in  colour  and  other  respects 
throughout  its  distribution,  excepting  that  on  both  sides  of  the 
Pyrenees  it  has  a  very  distinct  sporadic  variety  called  Andorrica 
or  microporus.  This  variety  occurs  here  and  there,  together 
with  the  type-form  sometimes  in  colonies  (pp.  26-30  and  86). 
Bulimus  detritus  though  more  restricted  in  geographical  range 
is  a  much  more  variable  form.  It  exhibits  great  variations  in 
colour,  form,  and  size,  and  as  Coutagne  well  insists,  these  are 
independent  of  each  other.  Foreshadowing  the  methods  of 
factorial  analysis  he  suggests  that  distinctions  in  each  respect, 
the  "modes"  as  he  calls  them,  should  be  denoted  by  a  letter, 
or  if  desired,  by  a  name,  and  the  several  combinations  of  differ- 
ences might  thus  be  most  logically  and  usefully  expressed.  Of 
such  combinations  he  says  there  are  at  least  18,  all  of  which  can 
be  found.  The  whole  possible  series  does  not  necessarily  occur 
in  the  same  place,  and  various  localities  are  characterised  by 
the  presence  or  absence  of  certain  of  the  combinations  as  Cou- 
tagne calls  them,  and  by  the  relative  frequency  with  which  they 
occur.  The  ideas  thus  enunciated  are  much  in  advance  of  the 
ordinary  practice  of  systematists,  who  give  names  to  forms  which 
are  nothing  but  accidental  combinations  of  factors,  just  as  the 
horticulturists  for  practical  reasons  give  names  to  similar  com- 
binations, which  as  we  now  know  are  merely  specially  noticeable 
terms  in  a  long  series  of  possibilities.  In  each  case  it  is  rather 
the  factors  which  should  be  named  than  the  forms  which  are 
constituted  by  their  casual  collocation.  In  this  special  example 
of  Bulimus  detritus  the  18  forms  are  made  by  the  combinations 
of  three  pairs  of  independent  factors.  Besides  these  combina- 
tions which  may  occur  anywhere  or  almost  anywhere  in  the  dis- 
tribution there  are  two  more  distinct  local  forms,  each  of  which 
is  regarded  by  Coutagne  as  probably  constituting  a  fresh  "  mode," 
perhaps  compatible  with  the  others. 


VARIATION  AND  LOCALITY  127 

Helix  striata  (Draparnauld)13  is  truly  polymorphic;  and  its 
various  forms  have  been  described  under  various  specific  names. 
It  abounds  in  the  calcareous  hills  of  Provence  and  Languedoc, 
disappearing  in  the  alluvial  lowlands  and  equally  in  the  upper 
levels  at  about  800-1,000  m.  From  this  district  it  extends 
through  regions  of  similar  altitude  over  a  great  part  of  France 
(details  given). 

Locard  in  his  monograph  of  this  group,  which  he  calls  col- 
lectively the  group  of  Helix  Heripensis,  tabulates  27  distinct 
named  forms.  The  characteristics  in  which  these  forms  differ 
have  been  reckoned  as  17,  and  as  several  of  these  vary  in  degree 
of  development,  the  number  of  modes  may  be  increased  to  109. 
For  practical  purposes  however  Coutagne  considers  that  the 
various  developments  of  7  characteristics  in  their  several  com- 
binations are  enough  to  express  the  various  forms,  and  he  gives 
examples  of  this  method  of  definition.  As  he  observes,  though 
names  may  be  required  to  define  the  modes,  no  one  need  be 
alarmed  at  that,  for  the  same  names  of  modes  will  be  applicable 
to  a  great  range  of  distinct  species,  and  the  formulae  expressing 
their  combinations  will  replace  the  varietal  names. 

This  particular  example  of  polymorphism  is  but  little  limited 
by  locality.  Occasional  colonies  present  some  special  physiog- 
nomy which  may  in  a  given  place  seem  almost  invariable,  though 
in  this  very  respect  the  colonies  found  elsewhere  may  be  highly 
variable,  but  such  limitations  are  exceptional  for  H.  striata. 

Some  distinct  and  obvious  susceptibilities  to  the  influence 
of  soil  and  climate  are  however  noticeable.  For  example  on 
siliceous  ground  the  shells  are  thinner,  while  on  calcareous  soils 
they  are  thicker ;  similarly  those  from  the  Northern  districts 
attain  a  larger  size  than  those  from  further  South.  Moreover 
those  subjected  to  curtailed  development,  whether  from  drought, 
heat  or  cold  often  show  a  shortening  of  the  spire.  In  contrast 
with  this  case  Coutagne  describes  the  varieties  of  Helix  caespitum, 
which  he  says  are  for  the  most  part  localised,  quoting  many  il- 
lustrative cases. 

Another  remarkable  case  in  which  locality  plays  a  curious 
part  is  provided  by  the  two  species  Helix  trochoides  and  pyra- 

18  As  to  the  synonymy  and  references  see  Coutagne,  p.  45. 


128  PROBLEMS  OF  GENETICS 

midata.  In  France  generally  they  are  distinct  enough  from 
each  other,  trochoides  being  smaller  and  having  a  characteristic 
keel.  Coutagne  says  that  after  having  collected  these  species 
from  more  than  a  score  of  localities  he  came  upon  a  colony  of 
trochoides  on  the  island  of  Pomegues  in  which  the  shells  were 
relatively  enormous,  most  of  them  having  only  a  slight  keel, 
and  a  few  none  at  all.  On  the  other  hand  he  received  a  con- 
signment of  pyramidata  from  four  localities  in  Sicily,  all  small, 
and  one  of  them  exactly  like  the  trochoides  from  Pomegues. 
Judging  by  the  samples  received  from  Sicily,  trochoides  is  there 
not  more  variable  than  it  is  in  Provence,  while  the  Sicilian 
pyramidata  is  protean. 

The  relations  of  the  two  species  Helix  nemoralis  and  hortensis 
provide  an  illustration  of  another  kind  of  manifestation  of  local 
peculiarity.  H.  hortensis  and  nemoralis  as  usually  met  with, 
are  two  very  distinct  forms.  H.  hortensis  is  smaller  and  duller, 
and  its  peristome  is  white.  H.  nemoralis  is  larger  and  more  shiny, 
and  its  peristome  is  brown.  In  several  anatomical  points, 
moreover,  especially  in  the  shape  of  the  dart,  there  are  great 
differences.  For  a  full  account  of  these  peculiarities  of  the  two 
forms  and  a  discussion  of  their  inter-relations  the  reader  is  re- 
ferred to  the  elaborate  work  of  A.  Lang14  who  has  studied  them 
extensively  and  has  also  succeeded  in  experimentally  raising 
hybrids  between  them.  These  hybrids  were  in  a  slight  degree 
fertile  with  both  the  parent  species,  but  up  to  the  time  of  pub- 
lication no  young  had  been  reared  from  hybrids  inter  se. 

Coutagne  describes  the  result  of  collections  made  in  62 
French  localities.  Some  had  exclusively  hortensis,  some  ex- 
clusively nemoralis,  and  in  some  the  two  were  found  in  associ- 
ation. He  gives  details  of  five  of  these  collections  from  which  I 
take  the  following  summary  of  the  more  essential  facts,  omitting 
much  that  is  almost  equally  significant. 

Locality  A ,  near  Honfleur.  Both  forms  present,  each  sharply 
and  normally  distinguished,  without  any  intermediates.  They 

14  A.  Lang,  Die  Bastarde  von  H.  hortensis  Mutter  H.  nemoralis  L.  Jena,  G. 
Fischer,  1908;  with  a  fine  coloured  plate  showing  the  varieties  of  the  species  and 
their  hybrids. 


VARIATION  AND  LOCALITY  129 

are  thus  found  in  many  places.  Coutagne  instances  Miiller's 
observations  in  Denmark,  his  own  series  from  the  Jura,  etc. 

Locality  B.  Vonges  (Cote  d'Or),  242  hortensis  taken  at  ran- 
dom, showed  128  with  light  peristomes  (either  more  or  less  pinkish 
or  quite  white)  and  114  with  dark  brown  peristomes;  together 
with  26  nemomlis  all  with  the  usual  brown  peristomes. 

Of  the  hortensis  50  were  in  ground-colour  opalescens  and  I 
roseus;  and  in  shape  5  were  umbilicatus. 

Locality  C,  about  3  kilometres  from  B.  There  were  found  35 
hortensis ,  of  which  20  had  light  peristomes  and  15  brown;  to- 
gether with  7  nemoralis. 

Of  the  hortensis  none  were  opalescens;  18  were  roseus  and  none 
has  the  shape  of  umbilicatus. 

Locality  D,  about  1,200  metres  from  B.  147  hortensis,  of 
which  4  had  light  peristomes  and  143  had  brown.  No  nemoralis 
were  found. 

None  of  the  hortensis  were  opalescens  or  roseus,  but  30  were 
umbilicatus. 

In  these  localities  intermediates  of  every  grade  existed  be- 
tween the  well-characterised  opalescens,  roseus,  or  umbilicatus, 
and  the  other  forms,  but  there  were  no  intergrades  between  the 
other  nemoralis  and  the  smaller  hortensis,  about  which  there 
was  no  hesitation.  In  the  next  locality  a  very  different  state 
of  things  was  found. 

Locality  E.  Banks  of  the  Yvette  at  Orsay  (Seine-et-Oise). 
The  actual  numbers  are  not  given,  but  we  are  told  that  58  per 
cent,  were  hortensis,  33  per  cent,  nemoralis,  and  9  per  cent,  inter- 
mediate. As  at  Honfleur,  the  hortensis  had  white  peristomes,  and 
the  nemoralis  brown.  Coutagne's  visits  to  this  locality  were 
in  1878  and  1880,  and  he  calls  attention  to  the  fact  that  Pascal 
found  similar  intermediates  in  the  same  neighbourhood  in  1873. 

The  two  species,  in  Coutagne's  view,  when  they  occur  to- 
gether, can  generally  be  sorted  from  each  other  with  perfect 
confidence,  and  it  is  only  in  exceptional  localities  that  these 
intermediates  occur.  Whether  they  are  hybrids,  or  whether 
sometimes  the  species  in  their  variations  transgress  their  usual 
limitations  is  regarded  both  by  Coutagne  and  by  Lang  as  a 

10 


130  PROBLEMS  OF  GENETICS 

question  not  yet  answerable  with  certainty.  Coutagne  moreover 
lays  stress  on  the  fact  that  although  each  species  may  be  easily 
known  from  the  other  in  its  own  district,  yet  when  shells  from 
different  districts  are  brought  together  it  is  sometimes  impossible 
to  sort  them.  He  mentions  an  example  of  such  casual  inter- 
mixture occurring  under  natural  conditions  on  an  island  in  the 
Rhone,  to  which  it  may  well  be  supposed  that  floods  had  brought 
immigrants  from  miscellaneous  localities.  This  population  con- 
tained a  very  large  number  of  uncertain  specimens,  and  as  he 
says,  it  was  much  as  if  he  were  to  mix  the  shells  from  his  62  local- 
ities, after  which  it  would  certainly  be  impossible  to  separate 
the  two  species  again.15 

Further  evidence  is  given  in  the  same  treatise  as  to  other 
examples  of  polymorphism,  especially  in  the  genus  Anodonta,  of 
which  Locard  made  251  species  for  France  alone.  Here  again 
are  cases  like  those  already  given,  and  many  forms  or  "modes" 
are  found  restricted  to  special  localities,  while  occasionally 
in  the  same  locality  dissimilar  forms  are  found,  collectively 
forming  a  colony,  without  intermediates. 

Taken  as  a  whole  the  evidence  shows  the  following  conclusions 
to  be  true.  Local  races,  whether  of  animals  or  plants,  may  be 
distinguished  by  characters  which  we  are  compelled  to  regard 
as  trivial,  or  again  by  features  of  such  magnitude  that  if  they 
were  known  to  us  only  as  the  characteristics  of  a  uniform  species 
they  would  certainly  be  assumed  without  hesitation  to  be  essential 
for  its  maintenance.  Local  forms  may  be  sharply  differentiated 
from  the  corresponding  populations  of  other  localities  or  they 
may  be  connected  with  them  by  numbers  of  intermediates. 
Not  rarely  also  we  find  a  fact  which  has  always  seemed  to  me  of 
special  significance,  that  the  peculiarity  of  the  local  population 
or  colony  may  show  itself  in  a  special  liability  to  variation,  and 
this  variability  may  show  itself  in  one  of  many  degrees,  either 
in  the  constant  possession  of  a  definite  aberration,  in  a  dimor- 
phism, or  in  an  extreme  polymorphism. 

At  this  stage  attention   should    be  called  to  two  points. 

15  With  this  evidence  compare  that  given  by  A.  Delcourt  in  his  valuable 
papers  lately  published  relating  to  the  variations  of  Notonecta.  See  especially 
Bull.  Set.  Fr.  Belg.,  1909,  XLIII,  p.  443;  and  C.  R.  Soc.  Biol.,  1909,  LXVI,  p.  589. 


VARIATION  AND  LOCALITY  131 

First,  that  when  the  details  of  the  geographical  distribution  of 
any  variable  species  are  studied  in  that  thorough  and  minute 
fashion  which  is  necessary  for  any  true  knowledge  of  the  inter- 
relations of  the  several  forms,  the  conception  of  a  species  invented 
by  the  popular  expositions  of  Evolution  under  Selection  is  found 
to  be  rarely  if  ever  realised  in  nature. 

A  species  in  this  generalised  sense  is  an  aggregate  of  indi- 
viduals, none  exactly  alike,  but  varying  round  a  normal  type, 
the  characters  of  which  are  fixed  in  so  far  as  they  are  adapted  to 
environmental  exigency.  In  nature,  however,  the  occurrence  of 
the  varieties,  and  even  the  occurrence  of  the  variability  is 
sporadic.  In  one  place  a  population  may  be  perfectly  uniform. 
In  another  it  may  be  again  uniform  but  distinct.  In  others 
the  two  forms  may  occur  together,  sometimes  with  and  sometimes 
without  intergrades.  In  some  localities  a  sporadic  variety 
may  be  an  element  of  the  population,  persisting  through  long 
periods  of  time.  In  other  localities  there  may  be  several  such 
aberrations  occurring  together  which  are  absent  elsewhere. 

Secondly,  I  would  remind  the  reader  that  in  the  light  of  genetic 
analysis  we  know  that  intergrades,  when  they  do  occur,  cannot 
be  assumed  to  represent  conditions  through  which  the  species 
must  pass  or  has  passed  on  its  way  to  the  extreme  and  definite 
forms. 

Often,  perhaps  generally,  they  are  nothing  but  heterozygous 
forms,  and  often  also  they  are  conditions  corresponding  with  the 
presence  of  factors  in  their  reduction-stages. 

A  broad  survey  of  the  facts  shows  beyond  question  that  it 
is  impossible  to  reconcile  the  mode  of  distribution  of  local  forms 
with  any  belief  that  they  are  on  the  whole  adaptational.  Their 
peculiarities  are  occasionally  the  result  of  direct  environmental 
influence,  as  we  shall  hereafter  notice  in  certain  cases,  but  none 
can  attribute  such  sporadic  and  irregular  phenomena  to  causes 
uniformly  acting. 

Writers  on  systematics,  especially  those  of  former  generations 
often  conjecture  or  assert  that  local  distinctions  are  caused  by 
"differences  of  climate,  soil,  food,  etc.,"  in  vague  general  terms. 
It  is  usually  safe  to  assume  that  these  remarks  do  not  represent 


132  PROBLEMS  OF  GENETICS 

conclusions  drawn  from  actual  evidence,  for  only  rarely  can  they 
be  translated  into  more  precise  language.  So  thoroughly  have 
the  biological  sciences  become  permeated  with  the  belief  that  all 
distinctions  are  dependent  upon  adaptation,  that  the  mere 
existence  of  definite  distinctions  is  felt  by  many  to  be  sufficient 
ground  to  warrant  an  assumption  that  these  distinctions  are 
directly  or  indirectly  due  to  special  local  conditions.  For 
example,  Dr.  J.  A.  Allen,  who  has  done  so  much  careful  and  valu- 
able work  in  delimiting  the  local  forms  of  the  United  States 
fauna,  writes  of  the  Ground  Squirrels  (Tamias)16  as  follows: — 

"From  the  extreme  susceptibility  of  this  plastic  group  to 
the  influences  of  environment,  it  is  one  of  the  most  instructive 
and  fascinating  groups  among  North  American  mammals.  No 
one  can  doubt  its  comparatively  recent  differentiation  from  a 
common  stock,  and  its  dispersion  from  some  common  centre. 
Whether  the  type  originated  at  some  point  in  North  America, 
or  in  the  Northern  part  of  Eurasia,  it  is  perhaps  idle  to  speculate, 
but  that  it  has  increased,  multiplied,  spread,  and  become  differ- 
entiated to  a  wonderful  degree  in  North  America  is  beyond 
question;  as  it  is  found  from  the  Arctic  regions  to  the  high 
mountain  ranges  of  Central  Mexico,  and  has  developed  some 
twenty  to  thirty  very  palpable  local  phases." 

"Some  of  them  easily  take  rank  as  species,  others  as  sub- 
species. Probably  a  more  striking  illustration  of  evolution  by 
environment  cannot  be  cited." 

He  proceeds  to  point  out  that  the  habits  of  these  creatures 
are  such  as  lead  to  isolation.  This  may  well  be  admitted,  and 
indeed  no  exception  can  possibly  be  taken  to  the  passage  as  a 
whole,  save  in  the  one  respect  that  there  is  no  real  proof  that 
the  local  diversity  is  due  to  "evolution  by  environment"  or  an 
indication  of  "susceptibility  to  the  influences  of  environment." 

Dr.  Allen  does  indeed  adduce  the  fact  that  California  "ex- 
tending through  800  miles  of  latitude,  with  numerous  sharply 
contrasted  physiographic  regions,  has  apparently  no  less  than 
six  strongly  differentiated  forms,  while  the  region  east  of  the 
Rocky  Mountains  from  a  little  below  the  northern  boundary  of 

"  Allen,  J.  A.,  Bull.  Amer.  Mus.  N.  H.,  Ill,  1891,  pp.  51-54. 


VARIATION  AND  LOCALITY  133 

the  United  States  northward  to  the  limit  of  trees — a  slightly 
diversified  region  of  at  least  ten  times  the  area  of  California — has 
only  one"!  But  when  one  comes  to  ask  how  the  various  forms 
are  adaptational,  and  how  the  influences  of  environment  have 
led  to  their  production,  only  conjectures  of  a  preliminary  and 
tentative  character  could  be  expected  in  reply.  Desert  forms 
are  no  doubt  pallid  as  in  so  many  instances,  and  forest  forms  are 
more  fully  coloured,  and  we  may  readily  enough  accept  such  facts 
as  indications  of  a  connection  between  bodily  features  and  the 
conditions  of  life,  but  further  than  that  no  one  can  go;  so  that 
when  we  find  size,  length  of  ears  or  of  tail,  the  number  of  dorsal 
stripes,  the  pattern  of  the  colours,  not  to  speak  of  differences  in 
the  pigments  themselves,  all  exhibiting  large  modifications,  we 
cannot  refer  these  peculiarities  to  the  causation  of  environmental 
difference,  save  as  a  simple  expression  of  faith.  I  incline  far 
more  to  agree  with  Gulick  who,  after  years  of  study  of  the  local 
variations  of  the  Achatinellidae,  came  to  the  conclusion  that  it 
was  useless  to  expect  that  such  local  differentiation  can  be 
referred  to  adaptation  in  any  sense.17  Even  the  most  convinced 
Selectionist  must  hesitate  before  such  facts  as  those  related  by 
A.  G.  Mayer  regarding  the  distribution  of  Partula  otaheitana,  one 
of  these  Achatinellidae.  The  island  of  Tahiti  has  been  scored 
by  erosion  so  that  a  series  of  separated  valleys  radiate  to  the  coast. 
From  four  successive  valleys  Mayer  collected  the  species,  and 
found  that  in  the  first  (Tipaerui)  valley  all  the  shells  were 
dextral  (115,  containing  73  young);  in  the  second  valley 
.(Fautaua)  54  per  cent,  of  adults  and  55.5  per  cent,  of  the  young 
contained  were  sinistral;  in  the  third  valley  (Hamuta)  69  per 
cent,  of  adults  and  73  per  cent,  of  young  contained  in  them  were 
sinistral;  and  lastly,  in  the  fourth  valley  (Pirae)  all  the  shells 
(131,  containing  62  young)  were  sinistral.18  In  connection 
with  these  observations  I  may  mention  the  fact  that  in  a  certain 
pond  in  the  North  of  England19  the  sinistral  form  of  Limnaea 

17  J.  T.  Gulick,  Evolution,  Racial  and  Habitudinal,  Carnegie  Institution,  Pub- 
lication No.  25,  1905. 

18  A.  G.  Mayer,  Mem.  Mus.  Comp.  Anat.  Harvard,  Vol.  XXVI,  1902,  p.  117. 
From  the  tables  given  I  cannot  ascertain  the  actual  numbers  from  the  two  inter- 
mediate valleys,  but  they  were  considerable. 

19  To  which  I  was  very  kindly  guided  by  Mr.  C.  T.  Trechmann. 


134  PROBLEMS  OF  GENETICS 

peregra  has  been  known  to  occur  for  about  fifty  years.  Visiting 
it  lately  I  found  the  left-handed  shells  to  be  about  3  per  cent,  of 
the  population.  The  species  is  the  commonest  British  fresh- 
water shell,  but  left-handed  specimens  are  exceedingly  rare. 
Will  anyone  ask  us  to  suppose  that  the  persistence  of  a  percentage 
of  this  rarity  in  the  same  place  is  an  indication  of  some  specially 
favouring  circumstance  in  the  waters  of  that  pond?  It  is  a 
horse-pond  to  all  appearances  exactly  like  any  other  horse-pond ; 
and  I  believe  that  in  perfect  confidence  we  may  accept  the 
suggestion  of  common  sense,  which  teaches  us  that  there  is 
nothing  particular  in  the  circumstances  which  either  calls  such 
varieties  into  existence  or  contributes  in  any  direct  way  to  their 
survival.  Had  the  phenomenon  of  local  variation  been  studied 
in  detail  before  Darwin  wrote,  the  attempt  to  make  selection 
responsible  for  fixity  wherever  found,  could  never  have  been 
made.  The  proposition  that  not  only  the  definiteness  of  local 
forms  but  their  variability  also  is  sporadic,  can  be  established 
by  countless  illustrations  taken  from  any  group  of  either  the 
animal  or  the  vegetable  kingdoms.  Only  exceptionally  can  the 
fixed  differences  be  even  suspected  of  contributing  to  adaptation, 
and  sporadic  variability,  which  is  a  no  less  positive  fact,  must 
manifestly  lie  outside  the  range  of  such  suspicions.  It  is  open 
to  any  one  to  suggest  speculatively  that  the  persistence  of 
special  varieties  or  of  special  variability  in  special  places  is  an 
indication  that  in  those  places  the  conditions  of  life  are  such 
that  the  forms  in  question  are  tolerated  though  elsewhere 
the  same  types  are  exterminated;  but  that  consideration,  even 
if  it  could  be  proved  to  be  well  founded,  is  not  one  which  lends 
much  force  to  the  thesis  that  definiteness  of  type  is  a  consequence 
of  Natural  Selection.  On  the  contrary,  recourse  to  such  reason- 
ing implies  the  inevitable  but  very  damaging  admission  that 
the  stringency  of  Selection  is  frequently  so  far  relaxed  that  two 
or  more  equally  definite  forms  of  the  same  species  can  persist 
side  by  side.  There  is  no  doubt  that  this  is  the  simple  truth, 
but  when  once  that  truth  is  perceived  it  is  useless  to  invoke  the 
control  of  Selection  as  the  factor  to  which  definiteness  of  type 
in  general  must  be  referred. 


VARIATION  AND  LOCALITY  135 

The  genetic  relations  of  local  forms  to  each  other  cannot  in 
the  absence  of  actual  breeding  experiments  be  often  ascertained. 
Standfuss  formerly  enunciated  as  a  general  principle  that  when 
two  forms  co-exist  in  the  same  locality  and  are  able  to  interbreed, 
they  do  not  produce  intermediates ;  but  that  when  the  forms  are 
geographically  separated  as  local  races,  crosses  between  them 
result  in  a  series  of  intermediates.20  In  this  aphorism  there  is  a 
good  deal  of  truth,  but  if  in  the  light  of  Mendelian  principles  we 
examine  the  two  statements  we  see  now  that  the  first  is  in  reality 
only  another  way  of  saying  that  the  distinctness  of  an  aberrational 
form  co-existing  with  another  is  due  to  segregation,  accompanied 
by  some  degree  of  dominance  of  one  type.  Whether,  however, 
one  geographically  isolated  race  will  give  intermediates  when 
bred  with  another  must  depend  entirely  on  the  genetic  physiology 
of  the  special  case,  and  no  general  rule  can  be  laid  down.  It 
may  well  be  that,  inasmuch  as  the  distinctness  of  the  variety  is 
maintained  by  isolation,  the  difference  in  factorial  composition 
between  it  and  the  representative  form  in  another  area  is  neither 
simple  nor  sharp;  but  when  two  varieties  co-exist,  though  inter- 
breeding, it  is  now  clear  that  their  differences  must  depend  on 
the  segregation  of  simple  factors.  Plainly  such  aberrations  may 
in  one  place  co-exist  with  another  type,  and  elsewhere  be  sep- 
arated from  it  as  local  races. 

Excellent  illustrations  of  these  two  stages  in  evolution  are 
provided  by  the  melanic  varieties  of  British  Lepidoptera.  The 
fact  that  black  or  blackish  varieties  of  many  species  especially 
of  Geometridae  have  come  into  existence  in  recent  years  is  well 
known  to  British  collectors,  and  it  is  not  in  dispute  that  they 
have  in  several  instances  replaced  the  older  type  more  or  less 
completely  in  certain  districts.  In  the  year  1900  the  Evolution 
Committee  of  the  Royal  Society  instituted  a  collective  inquiry 
as  to  the  contemporary  distribution  of  these  dark  varieties.  As 
the  change  had  happened  within  living  memory  and  had  greatly 
progressed  in  recent  years  it  was  hoped  that  a  record  of  the 
existing  distribution  would  serve  as  a  point  of  departure  for 
future  comparison.  The  records  thus  obtained  were  tabulated 

w  Standfuss,  Handbuch  d.  palaarkt  Gross-schmet,  1896,  p.  321. 


136  PROBLEMS  OF  GENETICS 

by  Mr.  L.  Doncaster.21  From  that  account  and  from  the  state- 
ments n  Barrett's  British  Lepidoptera22  this  description  of  some 
of  the  more  notable  cases  is  taken. 

The  most  striking  and  familiar  case  is  that  of  Amphidasys 
betularia,  of  which  only  the  ordinary  type  was  known  in  any 
locality  until  about  1848-1850,  when  the  totally  black  var. 
doubledayaria  first  appeared  in  the  neighbourhood  of  Manchester. 
This  black  form  was  subsequently  recorded  in  Huddersfield 
between  1860  and  1870;  Kendal  about  1870;  Cannock  Chase, 
1878;  Berkshire,  1885;  Norfolk,  Essex  and  Cambridge  about 
1892;  Suffolk,  1894;  London,  1897.  For  the  Southern  Counties 
of  England,  except  in  the  London  district,  there  are  still  very  few 
records.  It  cannot  of  course  be  asserted  positively  that  the 
variety  spread  from  its  place  of  first  appearance  into  the  other 
localities,  and  that  it  did  not  arise  de  novo  in  them,  but  there 
can  be  little  doubt  that  the  process  was  one  of  colonisation. 
On  the  European  Continent  the  first  records  are  from  Hanover 
in  1884,  Belgium  1886  and  1894,  Crefeld  188-,  Berlin  1903, 
Dresden  about  the  same  date. 

As  regards  the  increase  of  the  variety  we  have  the  fact  that 
in  Lancashire,  Cheshire  and  the  West  Riding  of  Yorkshire  the 
black  is  now  the  prevalent  form ;  and  in  some  places,  as  for  example, 
Huddersfield,  the  black  alone  is  now  found,  though  it  was  un- 
known there  till  between  1860  and  1870.  About  1870  at  New- 
port, Monmouth,  the  two  forms  were  in  about  equal  numbers, 
but  a  few  years  later  the  type  had  almost  vanished.  Similarly 
in  Crefeld,  where  the  black  form  was  still  very  rare  in  the  eighties, 
it  now  forms  about  50  per  cent,  of  the  population.  In  the 
London  district  the  black  remains  scarce  and  at  the  date  of  the 
report  it  was  still  very  scarce.  From  Ireland  there  is  only  one 
record  and  there  are  hardly  any  from  Scotland. 

Boarmia  repandata  is  another  species  which  is  behaving  in  a 
somewhat  similar  way.  Unlike  betularia,  however,  the  species 
is  a  variable  one,  and  has  several  colour-forms,  amongst  them 
the  banded  var.  conversaria,  and  many  others.  In  addition 

21  Ent.  Rec.,  XVIII,  No.  7,  1906. 

28  This  evidence  was  largely  collected  by  Mr.  G.  T.  Porritt,  who  has  given 
much  attention  to  the  subject. 


VARIATION  AND  LOCALITY  137 

to  these  there  is  a  black  form  in  the  North  of  England  which 
seems  to  be  spreading.  In  Huddersfield  the  black  was  first 
recorded  in  1888,  and  in  1900  20-25  per  cent,  were  black.  At 
Rotherham  the  black  or  very  dark  are  now  prevalent  and  have 
increased  in  the  last  15  years.  From  the  Midlands,  East  Anglia 
and  Southern  Counties  the  returns  show  only  the  light  and 
medium  forms. 

Of  Odontoptera  bidentata  several  intergrading  dark  forms  exist, 
and  these  are  found  exclusively  in  the  North  and  the  Midlands. 
Unicolorous  blacks  have  been  found  recently  in  the  Lancashire 
mosses  and  at  Wakefield.  At  Huddersfield  50  years  ago  the 
light  forms  were  prevalent,  but  now  a  rather  dark  brown,  not 
infrequently  suffused  with  black,  is  the  commonest.  In  Southern 
Counties  only  light  forms  are  known. 

Phigalia  pilosaria  in  South  England  is  always  light,  but  in 
the  North  the  prevalent  form  is  darker.  About  35  years  ago 
a  form  with  unicolorous  sooty  fore-wings  and  dull  grey  hind 
wings  was  first  seen  in  Yorkshire  and  a  similar  form  is  now  taken 
regularly  in  South  Wales. 

In  the  following  cases  the  dark  varieties  were  found  originally 
only  in  the  South. 

Boarmia  rhomboidaria  gave  rise  about  40  years  ago  to  a  uni- 
colorous smoky  variety  called  perfumaria.  This  was  at  first 
peculiar  to  the  London  district,  but  it  has  since  been  taken  in 
Birmingham  and  other  large  cities.  More  lately  coal-black 
specimens  have  been  found  at  Norwich,  and  others  similar  but 
hardly  so  dark  were  taken  in  the  South  of  Scotland  and  at 
Cannock  Chase. 

Eupithecia  rectangulata  is  a  similar  case.  Formerly  the 
light  forms  were  prevalent  but  within  sixty  years  they  have 
almost  entirely  been  replaced  in  the  South  of  London  by  a  nearly 
black  form. 

Tephrosia  (Boarmia)  consortaria  and  Tephrosia  consonaria  are 
exceptionally  interesting,  for  they  have  both  given  off  dark 
forms  in  the  same  wood  near  Maidstone,  which  is  far  from  the 
usual  "centres  of  melanism."  They  were  discovered  in  this 
locality  by  Mr.  E.  Goodwin.  That  of  consortaria  is  a  dark 


138  PROBLEMS  OF  GENETICS 

grey,  but  that  of  consonaria  is  a  full  black,  and  nothing  like 
either  has  been  found  anywhere  else. 

These  examples  are  all  taken  from  the  Geometridae  but 
others,  though  of  a  less  conspicuous  kind,  could  be  given  from 
the  Noctuidae  or  the  Micro-Lepidoptera.  Acronycta  psi,  for 
instance,  has  a  suffused  form  which  is  believed  to  be  becoming 
more  frequent  in  the  London  district.  Polia  chi  has  two  dark 
forms,  olivacea,  a  yellowish  grey  with  dark  markings,  and  suffusa 
which  is  a  darker,  blackish-slate  colour.  Both  occur  in  the  North 
of  England,  sometimes  together,  sometimes  separately,  or  mixed 
with  the  type  and  many  intermediates.  The  distribution  is 
peculiarly  irregular.  At  Huddersfield,  where  the  very  dark  form 
appeared  suddenly  about  1890,  some  30  per  cent,  are  said  to  be 
now  dark  and  about  6-7  per  cent,  very  dark,  but  at  Saddleworth, 
12  miles  away,  only  the  pale  forms  occur. 

Several  questions  of  interest  arise  in  regard  to  this  evidence. 
This  progressive  Melanism  has  arisen  in  certain  families  only, 
and  may  be  confined  to  certain  species  only,  within  those  families. 
As  in  almost  all  other  examples  in  which  variation  has  been  much 
observed,  its  incidence  is  capricious  and  specific.  A  collateral 
line  of  inquiry  relates  to  the  degree  of  discontinuity  which  the 
variation  manifests.  Here  again  there  is  no  rule.  Generally 
speaking,  in  A.  betularia,  to  take  the  case  most  fully  studied,  the 
variation  is  discontinuous.  Real  intermediates  between  betu- 
laria and  doubledayaria  are  in  most  localities  absent  or  rare. 
The  black  spots  of  betularia  may  often  be  larger  or  more  numerous 
than  in  the  normal,  but  this  variation  has  nothing  to  do  with 
doubledayaria,  and  is  not  an  intermediate  stage  towards  it, 
though  sometimes  wrongly  so  described.  Doubledayaria  owes  its 
characteristic  appearance  to  a  factor  which  blurs  the  surface 
of  the  wings  with  a  layer  of  black.  Sometimes  this  blurring  is 
slighter  than  in  the  real  doubledayaria,  and  these  forms  are  real 
intermediates.  Occasionally  the  forewings  alone  are  thus  blurred. 
These  intermediates  are  clearly  due  to  reduction-stages  of  the 
doubledayaria  factor,  and  are  related  to  it  as  a  blue  mouse  is  to 
a  black,  or  a  dutch  rabbit  to  a  self-colour.  It  cannot  positively 
be  asserted  that  the  full  doubledayaria  existed  before  the  inter- 


VARIATION  AND  LOCALITY  139 

mediate,  but  it  almost  certainly  did.  In  certain  places  as  for 
instance  in  Belgium,  there  is  evidence  that  intermediates  have 
at  various  times  been  fairly  abundant,  but  they  have  never  be- 
come common,  nor  are  they  known  to  exist  in  the  absence  of 
doubledayaria.  When  the  black  variety  and  the  light  type  breed 
together  they  do  not  usually  have  intermediates  among  their 
offspring,  and  the  evidence  is  consistent  with  the  view  that  the 
black  is  a  complete  dominant.  The  same  is  probably  true  of 
Tephrosia  consonaria. 

In  some  of  the  other  species  we  know  that  the  darkest  forms 
did  not  appear  first.  For  example  in  Phigalia  pilosaria  and 
Boarmia  rhomboidaria  dark  forms  existed  and  are  believed  to 
have  increased  in  number  before  the  darkest  made  its  appearance. 
Hybernia  progemmaria  is  said  to  have  become  darker  gradually 
both  in  Cheshire  and  in  the  West  Riding,  and  a  uniformly  smoky 
variety  appeared  in  South  Yorkshire  less  than  45  years  ago  which 
has  spread  to  neighbouring  counties.  The  dark  medium  has 
become  the  commonest  form  in  Huddersfield  district,  where  the 
very  dark  variety  is  now  about  20  per  cent,  of  the  population, 
though  the  light  form  is  still  common. 

Taking  the  evidence  together  we  find  it  consistent  with  the 
view  that  dark  forms  have  appeared  sporadically,  in  some  species 
the  very  dark  appearing  first  and  intermediates  later,  in  others 
the  moderately  dark  came  first  and  the  darkest  later  in  time.  It 
is  practically  certain  that  the  change  has  in  general  come  about 
not  by  a  gradual  change  supervening  on  the  population  at  large, 
but  by  the  sporadic  appearance  of  dark  specimens  as  a  new  ele- 
ment in  the  population,  and  strains  derived  from  these  dark 
individuals  have  gradually  superseded  the  normal  type  more  or 
less  completely. 

If  it  could  be  shown  that  these  melanic  novelties  had  a  defi- 
nite advantage  in  the  struggle  for  existence  they  would  provide 
an  instance  of  evolution  proceeding  much  in  the  way  which 
Darwin  contemplated.  The  whole  process  would  differ  from 
that  conceived  by  him  as  the  normal  method  of  evolution  only 
in  so  far  as  the  change  has  come  about  with  great  rapidity  and 
in  some  instances  largely  by  the  appearance  and  success  of  dis- 


140  PROBLEMS  OF  GENETICS 

continuous  varieties.  The  question,  however,  must  be  asked 
whether  the  dark  form  can  reasonably  be  supposed  to  have 
an  advantage  by  reason  of  their  darkness.  Some  naturalists 
believe  that  the  darkness  of  the  colours  does  thus  definitely  con- 
tribute to  their  protection  by  making  the  insects  less  conspicuous 
and  thus  more  likely  to  escape  the  search  of  birds.  In  support 
of  this  view  it  may  be  pointed  out  that  it  is  in  the  manufacturing 
districts  of  Lancashire  and  Yorkshire,  and  again  in  the  London 
area  that  the  melanics  have  attained  their  greatest  development. 
Consistently  with  this  argument  also,  it  is  in  the  neighbourhood  of 
Crefeld  and  Essen,  the  black  country  of  Germany,  that  they  have 
chiefly  established  themselves  on  the  Continent,  and  Phigalia 
pilosaria  in  the  black  form  is  now  at  home  in  South  Wales.  Thus 
superficially  regarded,  the  evidence  looks  rather  strong,  but  it 
is  difficult  to  apply  the  reasoning  in  detail.  We  have  first  the 
difficulty  that  the  black  form  of  betularia  for  instance  has  estab- 
lished itself  in  thoroughly  rural  districts,  notably  near  King's 
Lynn  in  Norfolk,  and  in  the  neighbourhood  of  Kendal  and 
Windermere.  The  black  form  of  consonaria  and  the  dark 
consortaria  appeared  in  a  wood  near  Maidstone,  far  from  town 
smoke,  and  the  black  rhomb oidaria  was  first  found  at  Norwich, 
which,  as  towns  go,  is  clean.  Then  again  the  spread  of  the 
melanics  is  very  irregular  and  unaccountable.  The  black  pilo- 
saria is  found  both  in  the  West  Riding  and  in  the  Swansea 
district,  but  not  yet  elsewhere.  It  rapidly  increased  at  Hudders- 
field,  but  made  no  noticeable  progress  at  Sheffield  though  re- 
corded there  for  ten  years.  It  is  also  a  remarkable  fact  that  no 
similar  melanic  development  has  been  observed  in  America, 
and,  so  far  as  I  am  aware,  comparable  melanic  varieties  have  not 
appeared  on  the  European  continent  except  in  the  case  of  the 
few  sorts  which  possibly  may  have  come  from  England. 

The  whole  subject  is  beset  with  complications.  It  must 
not  be  forgotten  that  in  a  few  species  of  moths  there  is  an  obvious 
and  recognised  conformity  between  the  colours  of  the  perfect 
insect  and  that  of  the  soil  on  which  they  live,  comparable  with 
that  which  is  so  striking  in  the  case  of  some  Oedipodidae  and 
other  grasshoppers.  Of  this  phenomenon  the  clearest  example 


VARIATION  AND  LOCALITY  141 

is  Gnophos  obscurata,  which  is  a  most  variable  species  with  many 
local  forms.  Of  these  a  well-known  dark  variety  lives  on  the 
peaty  heaths  of  the  New  Forest  and  other  districts,  but  on  the 
chalk  hills  of  Kent,  Sussex  and  Surrey  various  light  varieties 
are  found,  of  which  one  is  a  bright  silvery  white,  very  near  in 
colour  to  the  colour  of  a  chalky  bank.  This  case  does  not  seem 
to  be  one  of  direct  environmental  action,23  for  Poulton  found  no 
change  induced  by  rearing  larvae  among  either  white  or  black 
surrounding  objects.  No  one  however  can  doubt  that  there  is 
some  indirect  connection  between  the  colour  of  the  ground  and 
that  of  the  moths. 

To  my  mind  there  is  a  serious  objection  to  the  theory  of  pro- 
tective resemblance  in  application  to  such  a  case  as  that  of  the 
betularia  forms,  which  arises  from  the  fact  that  the  black  double- 
dayaria  is  a  fairly  conspicuous  insect  anywhere  except  perhaps 
on  actually  black  materials,  which  are  not  common  in  any 
locality.  Tree  trunks  and  walls  are  dirty  in  smoky  districts  but 
they  are  not  often  black,  and  I  doubt  whether  in  the  neighbour- 
hood of  Rotherham,  for  instance,  which  is  one  of  the  great 
melanic  centres,  doubledayaria  can  be  harder  for  a  bird  to  find 
than  betularia  would  be.  After  all,  too,  many  of  the  species 
much  affected  are  not  urban  insects.  They  live  in  country 
places  between  the  towns,  and  the  general  tone  of  these  places 
even  in  Lancashire  and  the  West  Riding  is  not  very  different 
from  that  of  similar  places  elsewhere.  As  against  the  objection 
that  the  black  varieties  are  much  blacker  than  the  case  requires 
it  may  be  replied  that  we  know  nothing  of  the  senses  of  birds, 
and  that  perhaps  to  their  eyes  blackness  does  constitute  a  dis- 
guise even  though  the  surroundings  are  much  less  dark.  This  is 
undeniable,  but  recourse  to  such  an  argument  is  dangerous;  for 
if  the  sight  of  the  insect-eating  birds  is  so  dull  that  it  does  not 
distinguish  dark  things  from  dingy  grey,  we  cannot  subsequently 
regard  the  keen  sight  of  birds  as  the  sufficient  control  which  has 
led  to  the  minute  and  detailed  resemblance  of  many  insects  to 
their  surroundings.  Th^se  who  see  in  such  cases  examples  of 

»  Such  direct  action  has  of  course  been  proved  to  occur  in  the  case  of  several 
dimorphic  larvae  (e.  g.,  A.  betularia,  itself)  and  pupae. 


142  PROBLEMS  OF  GENETICS 

the  omnipotence  of  Selection  must  frequently  find  themselves 
in  this  dilemma. 

Taking  the  evidence  as  a  whole,  we  may  say  that  it  fairly 
suggests  the  existence  of  some  connection  between  modern  urban 
developments  and  the  appearance  and  rise  of  the  melanic  vari- 
eties. More  than  that  we  cannot  yet  affirm.  It  is  a  subject 
in  which  problems  open  up  on  every  side,  and  all  of  them  are 
profitable  subjects  for  investigation.  Unhappily  such  animals 
are  difficult  to  rear  successfully  in  captivity  for  many  generations, 
owing  to  their  extreme  liability  to  disease.  Not  the  least  in- 
teresting feature  of  the  melanics  is  the  fact  that  the  black  vari- 
eties provide  about  the  best  and  clearest  example  of  a  new  dom- 
inant factor  attaching  itself  to  a  wild  species  in  recent  times. 
None  of  the  cases  are  satisfactorily  recorded  or  analysed  as  yet, 
but  the  evidence  is  clear  that  doubledayaria  is  a  dominant  to  its 
type,  and  in  several  other  dark  varieties,  though  the  pigment 
deposited  is  not  black,  the  records  show  that  the  increased 
amount  of  the  pigment  almost  certainly  is  due  to  a  positive  factor. 
Of  this,  Hemerophila  abruptaria  is  a  good  example.24  There  are 
some  irregularities  in  the  results,  but  taken  together  they  leave 
little  doubt  that  the  dark  brown  variety  is  a  dominant  and  the 
light,  yellowish  brown  a  recessive. 

A  curious  parallel  to  the  rise  of  the  melanic  moths  in  England 
is  provided  by  the  case  of  the  Honey-creepers  or  Sugar-birds, 
in  certain  West  Indian  islands.25  These  birds  of  the  genus 
Coereba  (Certhiola)  range  from  Southern  Mexico  to  the  Northern 
parts  of  South  America  and  through  the  whole  chain  of  the  West 
Indian  islands  and  Bahamas  except  Cuba.  There  are  numerous 
local  forms,  and  many  of  the  islands  have  types  peculiar  to  them- 
selves, as  is  usual  in  such  cases.  Some  of  the  types  or  species 
range  through  several  islands,  but  according  to  Austin  Clark26 
no  island  has  more  than  one  of  them.  Cory27  reckoned  twelve 

24  See  Harris,  Proc.  Ent.  Soc.  London,  1904,  p.  Ixxii,  and  1905,  p.  Ixiii;  also 
Hamling,  Trans.  City  of  London  Ent.  Soc.,  1905,  p.  5. 

25 1  am  indebted  to  Mr.  Outram  Bangs  of  the  Harvard  Museum  for  calling  my 
attention  to  this  remarkable  case. 

26  Auk,  1889,  VI,  p.  219. 

27  Ann.  N.  Y.  Acad.  Sci.,  1878,  I,  p.  149. 


VARIATION  AND  LOCALITY  143 

such  species  within  the  Antillean  region.  They  are  small  birds 
about  the  size  of  a  nuthatch  with  a  general  colouring  of  black, 
yellow,  and  white.  From  the  island  of  St.  Vincent  the  Smith- 
sonian Institution  received  in  the  late  seventies  of  last  century 
several  completely  black  specimens  in  addition  to  two  of  the  usual 
type  of  colouring.  The  black  were  described  by  W.  N.  Lawrence 
as  atrata,  and  those  marked  with  the  usual  yellow  and  white 
were  called  saccharina.  The  collector  (Mr.  F.  A.  Ober)  reported 
that  the  black  form  was  common,  and  that  the  saccharina  form 
was  rarer.  Lawrence  remarks,  "Had  there  been  only  a  single 
example  (of  the  black  form)  I  should  have  considered  it  as  prob- 
ably a  case  of  abnormal  colouring,  but  it  seems  to  be  a  represent- 
ative form  of  the  genus  in  this  island."  28  There  is  of  course  no 
doubt  of  the  correctness  of  the  view  taken  by  Austin  Clark  that 
" atrata"  is  a  black  variety.  The  black  bird  is  in  every  respect, 
other  than  colour,  identical  with  saccharina,  and  it  is  even  possible 
to  detect  a  greenish  colour  in  the  areas  which  would  normally  be 
yellow,  showing  plainly  enough  the  yellow  pigment  obscured  by 
the  black. 

We  have  next  the  interesting  fact  that  like  our  melanic  moths 
the  dark  form  is  replacing  the  "type."  At  the  time  of  Ober's 
visit  the  type  was  already  in  a  minority,  but  now  it  is  nearly 
or  perhaps  actually  extinct,  though  the  black  form  is  one  of  the 
commonest  birds  on  the  island.  Austin  Clark  found  no  specimen 
when  he  collected  there  in  1903-4,  though  formerly  it  was  not 
uncommon  in  the  vicinity  of  Kingston  and  in  the  immediate 
windward  district  of  St.  Vincent. 

The  Grenadines  are  geographically  just  south  of  St.  Vincent, 
though  separated  by  a  deep  channel.  In  these  islands  no  black 
forms  have  yet  been  taken,  but  Grenada,  the  next  island  to  the 
south,  has  both  normals  and  blacks.  There  are  trifling  dif- 
ferences of  size  between  the  Grenada  birds  and  those  from  St. 
Vincent,  the  Grenada  specimens  being  slightly  smaller  and  for 
this  reason  they  have  received  distinct  names,  the  form  marked 
with  yellow  and  white  being  called  Godmani  (Cory)  and  the  black, 
Wellsi  (Cory),  but  this  merely  introduces  a  useless  complication. 

*  Ann.  N.  Y.  Acad.  Sci.,  1878,  I,  p.  149. 


144  PROBLEMS  OF  GENETICS 

There  is  evidence  that  in  Grenada,  as  in  St.  Vincent,  the  black  is 
gradually  ousting  the  original  type,  but  the  process  has  not  gone 
so  far  as  in  St.  Vincent.  Austin  Clark  very  properly  compares 
this  case  of  the  Sugar-birds  with  that  of  Papilio  turnus,  which  as 
is  well-known,  has  a  black  female  in  the  southern  parts  of  its  dis- 
tribution, in  addition  to  a  female  of  the  yellow  type,  but  in  the 
Northern  States  the  black  female  does  not  occur. 

During  the  present  year  P.  R.  Lowe,  who  lately  studied 
Coerebas  on  a  large  scale  in  the  West  Indies,  has  published  an 
important  paper  on  the  subject.29  He  calls  attention  to  the 
fact  that  Cory  recently  found  a  black  form  of  Coereba  on  Los 
Roques  Islands,  and  he  himself  discovered  another  on  the 
Testigos  Islands.  Both  localities  are  on  the  coast  of  Venezuela, 
far  from  St.  Vincent  and  Grenada.  The  whole  problem  is  thus 
further  complicated  by  the  fact  that  the  black  varieties  have, 
as  we  are  almost  driven  to  admit,  arisen  independently  in  remote 
places.  Improbable  as  this  conclusion  may  be,  it  is  still  more 
difficult  to  regard  all  the  black  forms  as  derived  from  one  source. 
For  first,  they  present  definite  small  differences  from  each  other; 
and  secondly  we  have  to  remember  a  consideration  of  greater 
importance,  that  the  very  fact  that  each  island  has  its  own  type 
must  be  accepted  as  proving  that  the  localities  are  effectively 
isolated  from  each  other,  and  that  migration  must  be  a  very 
rare  event. 

The  rarity  of  such  illustrative  cases  is,  I  believe,  more  ap- 
parent than  real.  It  is  probably  due  to  the  extreme  reluctance 
of  systematists  to  admit  that  such  things  can  be,  and  of  course 
to  the  almost  complete  absence  of  knowledge  as  to  the  genetic 
behaviour  of  wild  animals  and  plants.  Only  in  such  examples 
as  this  of  the  Coereba,  where  colour  constitutes  the  sole  differ- 
ence, or  that  of  the  moths  which  have  been  minutely  studied  by 
many  collectors,  does  the  significance  of  the  facts  appear.  The 
arrangement  of  catalogues  and  collections  is  such  that  much 
practical  difficulty  of  a  quite  unnecessary  kind  is  introduced.  For 
example,  in  this  very  case  of  Coereba,  I  find  the  British  Museum 
has  a  fine  series  from  Grenada  including  3  normals  and  1 1  black, 

29  Ibis,  1912,  pp.  523-8. 


VARIATION  AND  LOCALITY  145 

and  also  1 6  blacks  from  St.  Vincent.  If  the  black  specimens  from 
Grenada  were  put  with  the  normals  which  are  almost  certainly 
nothing  but  a  recessive  form  of  the  same  bird,  the  variation  would 
strike  the  eye  on  even  a  superficial  glance  at  the  drawer.  But 
following  the  notions  so  naively  expressed  in  the  passage  quoted 
above  from  W.  N.  Lawrence,  the  blacks  from  Grenada  are  put 
apart  together  with  the  other  blacks  from  St.  Vincent,  though 
two  of  them  were  shot  on  the  same  date  as  one  of  the  normals. 


CHAPTER  VII 

LOCAL  DIFFERENTIATION.     Continued 

OVERLAPPING  FORMS 

The  facts  of  the  distribution  of  local  forms  on  the  whole  are  con- 
sistent with  the  view  that  these  forms  come  into  existence  by  the 
sporadic  appearance  of  varieties  in  a  population,  rather  than  by 
transformation  of  the  population  as  a  whole.  Of  such  sporadic- 
ally occurring  varieties  there  are  examples  in  great  abundance, 
though  by  the  nature  of  the  case  it  can  be  but  rarely  that  we  are 
able  to  produce  evidence  of  a  previous  type  being  actually  super- 
seded by  the  variety.  When  the  two  forms  are  found  co-existing 
in  the  same  area  they  are  usually  recorded  as  one  species  if  inter- 
grades  are  observed,  and  as  two  species  if  the  intergrades  are 
absent.  On  the  other  hand  when  two  forms  are  found  occupying 
separate  areas,  when,  that  is,  the  process  of  replacement  is  com- 
pleted in  one  of  the  areas,  then  forthwith  each  is  named  separately 
either  as  species  or  subspecies.  Successive  observations  carried 
out  through  considerable  periods  of  time  would  be  necessary  to 
establish  beyond  question  that  the  history  proceeds  in  one  way 
rather  than  another.  Such  continuity  of  observation  has  for 
the  most  part  never  been  attempted.  The  kind  of  information 
wanted  has  indeed  only  been  lately  recognized,  and  really  critical 
collecting  is  a  thing  of  only  the  last  few  decades.  The  methods  of 
the  older  collectors,  who  aimed  at  bringing  together  a  few  typical 
specimens  of  all  distinct  forms,  are  of  little  service  in  this  class 
of  inquiry,  which  is  better  promoted  by  the  indiscriminate  col- 
lection of  large  numbers  of  common  forms  from  many  localities. 
When  this  has  been  done  on  a  comprehensive  scale  we  shall  be 
in  a  position  to  form  much  more  confident  judgments  as  to  the 
general  theory  of  evolution. 

Some  little  work  of  the  kind  has  however  been  done  and  the 
results  are  already  of  great  value.  Seeing  that  the  differenti- 
ation of  local  forms  is  only  made  possible  by  isolation,  it  neces- 

146 


OVERLAPPING  FORMS  147 

sarily  happens  that  the  collector  finds  one  form  in  one  locality 
and  another  in  a  distinct  locality,  and  there  is  no  evidence  as  to 
the  behaviour  which  the  two  representative  species  might  exhibit 
if  they  came  into  touch  with  each  other.  In  the  most  familiar 
examples  of  such  distinction  each  inhabits  an  island,  completely 
occupying  it  to  the  exclusion  of  any  other  similar  form.  It  can 
only  be  when  the  two  representative  species  occupy  parts  of  a 
continental  area  connected  with  each  other  by  regions  habitable 
for  the  organism  in  question,  that  there  is  a  chance  of  seeing  the 
two  forms  in  contact.  Often  also,  even  where  this  condition  is 
satisfied,  the  habits,  social  organisation,  or  some  other  special 
cause  may  act  as  a  barrier  which  prevents  the  distinguishable 
forms  from  ever  coming  into  such  complete  contact  as  to  inter- 
breed or  to  behave  as  a  genetically  continuous  race.  When 
genetic  continuity  is  ensured  by  a  constant  diffusion  of  the  popu- 
lation over  the  whole  area  which  they  inhabit  there  will  mani- 
festly be  no  formation  of  local  races.  The  practical  uniformity, 
for  example,  of  so  many  species  of  birds  which  inhabit  widely 
extended  ranges  of  Western  Europe  is  doubtless  maintained  by 
such  constant  diffusion.  When,  as  in  the  case  of  the  Falcons, 
many  localities  have  peculiar  forms,  the  fact  may  be  taken  as 
conclusive  evidence  that  there  is  little  or  no  diffusion;  and  when 
we  find  in  such  a  species  as  the  Goldfinch  that  in  spite  of  mi- 
gratory fluctuations  there  are  nevertheless  geographical  races 
fairly  well  differentiated,  it  may  similarly  be  inferred  that  these 
fluctuations  habitually  move  up  and  down  on  paths  which  do 
not  intermingle.  There  are  however  a  few  examples  of  animals, 
not  given  to  much  irregular  wandering,  which  occupy  a  wide  and 
continuous  range  of  diversified  country  and  are  differentiated  as 
local  races  in  two  or  more  districts,  though  the  distinct  races 
meet  in  intervening  areas.  Of  these  the  most  notorious  illus- 
tration which  has  been  investigated  with  any  thoroughness  is 
that  of  the  species  of  Colaptes  (Woodpeckers)  known  in  the  United 
States  as  Flickers.  The  study  of  the  variations  of  these  forms, 
made  by  J.  A.  Allen1  is  an  admirable  piece  of  work,  with  which 

1  J.  A.  Allen,  The  North  American  Species  of  the  Genus  Colaptes,  Considered  with 
Special  Reference  to  the  Relationships  of  C.  auratus  and  C.  cafer.  Bull.  Am.  Mus. 
Nat.  Hist.,  IV,  1892. 


148  PROBLEMS  OF  GENETICS 

every  student  of  variation  and  evolutionary  problems  should 
make  himself  familiar.  The  two  forms  with  which  we  are  most 
concerned  are  known  as  C.  auratus  and  C.  cafer,  and  are  very 
strikingly  different  in  appearance.  In  size,  proportions,  general 
pattern  of  colouration,  habits,  and  notes,  the  two  are  alike,  but 
they  differ  in  the  following  seven  respects  as  stated  by  Allen. 

Auratus  Cafer 

1.  Quills  yellow.  I.  Quills  red. 

2.  Male  with  a  black  malar  stripe.  2.  Male  with  a  red  malar  stripe. 

3.  Adult  female  with  no  malar  stripe.          3.  Adult  female  with  usually  a  brown 

malar  stripe. 

4.  A  scarlet  nuchal  crescent  in  both  sexes.       4.  No  nuchal  crescent  in  either  sex. 

5.  Throat  and  fore  neck  brown.  5.  Throat  and  fore  neck  grey. 

6.  Whole  top  of  head  and  hind  neck       6.  Whole  top  of  neck  and  hind  neck 

grey.  brown. 

7.  General  plumage  with  an  olivaceous       7.  General    plumage   with   a   rufescent 

cast.  cast. 

These  differences  are  illustrated  in  the  accompanying  coloured 
plate,  which  has  been  most  kindly  prepared  for  me  under  the  in- 
structions of  Dr.  F.  M.  Chapman  of  the  American  Museum  of 
Natural  History.  Before  going  further  it  is  worth  considering  the 
nature  of  these  differences  a  little  more  closely.  All  but  the  last 
are  large  differences  which  no  one  would  overlook  even  in  a  hasty 
glance  at  the  birds.  If  the  only  distinction  lay  in  the  colour  of  the 
quills  we  might  feel  fairly  sure  that  auratus  was  a  recessive  form 
of  cafer,  and  so  probably  it  is  in  this  respect.  Similarly  the  black 
malar  stripe  of  auratus  is  in  all  probability  recessive  to  the  red 
malar  stripe  of  cafer  and  I  imagine  the  pigments  concerned  are 
comparable  with  those  in  the  Gouldian  Finch  (Poephila  gouldiae) 
of  Australia.  Both  sexes  in  that  species  may  have  the  head  black, 
red,  or,  less  often,  yellow,  and  though  it  is  not  any  longer  in 
question  that  birds  may  breed  in  either  plumage,  I  believe  that 
the  young  are  always  black-headed  and  I  imagine  that  those 
which  become  red-headed  possess  a  dominant  factor  absent  from 
the  permanently  black-headed  birds.2  Yellow  as  a  recessive 

2  For  a  case  in  which  a  red-headed  female  X  a  black-headed  male  gave  a 
black-headed  female  and  a  red-headed  male,  see  Avian  Mag.,  N.  S.,  IV,  pp.  49 
and  329. 


OVERLAPPING  FORMS  149 

form  of  a  red  is  certainly  very  common,  but  red  and  black  as 
variants  of  the  same  pigment  are  less  usual.  In  the  Gouldian 
Finch  we  seem  to  have  a  case  where  a  pigment  can  assume  all 
three  forms.  It  would  be  interesting  to  know  whether  the  red 
of  the  malar  stripes  in  Colaptes  is  a  pigment  of  the  same  nature 
as  the  red  of  the  quills.  Both  in  Colaptes  and  in  Poephila 
gouldiae  I  have  seen  specimens  intermediate  between  the  black 
and  the  red,  and  the  appearance  of  the  part  affected  was  exactly 
alike  in  the  two  cases,  red  feathers  coming  up  among  the  black 
ones,  and  many  feathers  containing  both  red  and  black  pigments 
mixed  together. 

The  development  of  the  scarlet  nuchal  crescent  in  auratus 
and  the  absence  of  this  conspicuous  mark  in  cafer  constitute  from 
the  physiological  point  of  view  the  most  remarkable  pair  of  dif- 
ferences. When  the  red  crescent  is  not  formed,  the  feathers 
which  would  bear  it  are  exactly  like  the  rest,  and  no  special 
pigment  is  visible  in  them  which  one  can  regard  as  ready  to  be 
modified  into  red.  If  the  crescent  is  due  to  a  factor  it  must 
therefore  be  supposed  that  this  factor  has  the  power  of  modifying 
the  pigment  of  the  neck  in  one  special  place  alone.  Dr.  W.  D. 
Miller  called  my  attention  to  the  fact  that  a  similar  variation 
occurs  in  another  American  woodpecker,  the  Sapsucker,  Sphy- 
ropicus  varius? 

I  do  not  suggest  that  such  variations  are  without  parallel: 
indeed  in  P.  gouldiae  the  factor  which  turns  the  black  of  the  head 
into  scarlet  affects  one  special  region  of  the  black  only,  being 
sharply  distinct  from  the  unmodified  black  of  the  throat.  These 
regions  of  the  head  are  however  often  the  seat  of  special  colours 
in  birds.4  So  also  may  be  instanced  the  variety  of  the  Common 

8  The  other  variations  of  this  bird  are  also  interesting  and  important.  The 
normal  male  has  a  red  head  and  a  red  throat.  The  female  has  a  red  head  and  a 
white  throat,  but  varieties  of  the  female  are  known  with  a  black  head,  thus  again 
illustrating  the  change  from  black  to  red.  It  should  be  noted  that  this  is  not  a 
mere  retention  of  a  juvenile  character,  but,  as  the  birds  mature,  the  red  feathers 
come  up,  or  as  an  exception,  the  black.  There  is  also  a  western  species,  ruber, 
in  which  both  sexes  have  a  great  extension  of  red,  and  are  alike.  The  male  of 
nuchalis  intergrades  with  this  type,  but  the  female  does  not. 

4  Dr.  W.  Brewster,  for  example,  has  a  remarkable  specimen  of  the  Teal  (Nettion 
carolinense)  with  a  white  collar  strongly  developed  at  the  front  and  sides  of  the 
neck,  in  a  place  where  the  normal  has  no  such  mark. 


150  PROBLEMS  OF  GENETICS 

Guillemot  (Uria  troile)  which  has  a  white  line  round  the  eyes  and 
at  the  sides  of  the  head  where  the  normal  has  no  such  mark;  but 
this  line  is  formed  in  a  very  special  place,  the  groove  joining  the 
eye  to  the  ear,  whereas  the  feathers  of  the  nuchal  crescent  are 
not  ostensibly  distinguished  from  those  adjacent.5 

The  transposition  of  the  brown  and  the  grey  on  the  back  and 
front  of  the  neck  also  constitutes  a  very  remarkable  difference. 
If  either  grey  or  brown  depends  on  a  factor  then  it  must  be  sup- 
posed that  auratus  has  one  of  these  factors  and  cafer  the  other. 

From  these  several  considerations  it  is  quite  clear  that  if 
auratus  and  cafer  are  modifications  of  the  same  type  produced 
by  presence  or  absence  of  factors,  several  independent  elements 
must  be  concerned,  and  to  unravel  their  inter-relations  would  be 
most  difficult  even  if  it  were  possible  to  breed  the  types  under  ob- 
servation, which  is  of  course  quite  beyond  present  possibilities. 

The  distribution  of  the  two  is  as  follows.  On  the  east  side  of 
the  Continent  C.  auratus,  relatively  pure,  occupies  the  whole  of 
Canada  and  the  States  from  the  North  to  Galveston.  Westward 
it  extends  across  the  whole  continent  in  the  more  northern 
region  to  Alaska,  but  in  its  pure  form  it  only  reaches  down  the 
Pacific  coast  to  about  the  northern  border  of  British  Columbia. 
Its  southern  and  western  limit  is  thus  roughly  a  line  drawn  from 
north  of  Vancouver,  southeast  to  North  Dakota  and  then  south 
to  Galveston.  C.  cafer  in  the  comparatively  pure  form  inhabits 
Mexico,  Arizona,  California  (except  Lower  California  and  the 
opposite  coast),  central  and  western  Nevada,  Utah,  Oregon,  and 
is  bounded  on  the  east  by  a  line  drawn  from  the  Pacific  south  of 
Washington,  south  and  eastward  through  Colorado  to  the  mouth 

B  This  variety  is  spoken  of  as  the  Ringed  Guillemot  and  is  sometimes  regarded 
as  a  distinct  species  to  which  the  name  ringvia  was  given  by  Briinnich.  In  sup- 
port of  this  view  Dr.  William  Brewster,  to  whom  I  am  indebted  for  much  assist- 
ance in  regard  to  the  variation  of  birds,  called  my  attention  to  observations  of  his 
own  and  also  of  Maynard's,  that  the  ringed  birds  were  sometimes  mated  together, 
though  in  a  small  minority  (see  Brewster,  Proc.  Boston  Soc.  N.  H.,  XXII,  1883,  p. 
410).  It  would  however  be  possible  to  produce  many  instances  of  varieties  mated 
together  though  surrounded  by  a  typical  population  (e.  g.,  two  varying  Black- 
birds, Zoologist,  p.  2765;  two  varying  Nightjars,  ibid.,  p.  5278).  I  am  inclined  to 
believe  that  in  nature  matings  between  brothers  and  sisters  are  frequent  in  many 
species  of  animals,  and  that  the  production  of  sporadically  varying  colonies  is 
thus  greatly  assisted. 


OVERLAPPING  FORMS  151 

of  the  Rio  Grande  or  the  Gulf  of  Mexico.  Between  the  two 
lines  thus  roughly  denned  is  a  band  of  country  about  1 ,200-1 ,300 
miles  long  and  300-400  miles  wide,  which  contains  some  normal 
birds  of  each  type,  but  chiefly  birds  exhibiting  the  characters  of 
both,  mixed  together  in  various  and  irregular  ways.  Even  in 
the  areas  occupied  by  the  pure  forms  occasional  birds  are  re- 
corded with  more  or  less  indication  of  characteristics  of  the  other 
form,  but  within  the  area  in  which  the  two  forms  are  conterminous, 
the  mixed  birds  are  in  the  majority.  The  condition  of  these  birds 
of  mixed  character  is  described  by  Allen  as  follows : 

"As  has  been  long  known — indeed,  as  shown  by  Baird  in 
1858 — the  'intermediates'  or  'hybrids'  present  ever-varying 
combinations  of  the  characters  of  the  two  birds,  from  individuals 
of  C.  auratus  presenting  only  the  slightest  traces  of  the  char- 
acters of  C.  cafer,  or,  conversely — individuals  of  C.  cafer  present- 
ing only  the  slighest  traces  of  the  characters  of  C.  auratus — 
to  birds  in  which  the  characters  of  the  two  are  about  equally 
blended.  Thus  we  may  have  C.  auratus  with  merely  a  few  red 
feathers  in  the  black  malar  stripe,  or  with  the  quills  merely 
slightly  flushed  with  orange,  or  C.  cafer  with  either  merely  a  few 
black  feathers  in  the  red  malar  stripe,  or  a  few  red  feathers  at 
the  sides  of  the  nape,  or  an  incipient,  barely  traceable  scarlet 
nuchal  crescent.  Where  the  blending  of  the  characters  is  more 
strongly  marked,  the  quills  may  be  orange-yellow  or  orange-red, 
or  of  any  shade  between  yellow  and  red,  with  the  other  features 
of  the  two  birds  about  equally  blended.  But  such  examples  are 
exceptional,  an  unsymmetrical  blending  being  the  rule,  the  two 
sides  of  the  same  bird  being  often  unlike.  The  quills  of  the  tail, 
for  example,  may  be  part  red  and  part  yellow,  the  number  of 
yellow  or  red  feathers  varying  in  different  individuals,  and  very 
often  in  the  opposite  sides  of  the  tail  in  the  same  bird.  The 
same  irregularity  occurs  also,  but  apparently  less  frequently, 
in  the  quills  of  the  wings.  In  such  cases  the  quills  may  be  mostly 
yellow  with  a  few  red  or  orange  quills  intermixed,  or  red  with  a 
similar  mixture  of  yellow.  A  bird  may  have  the  general  coloura- 
tion of  true  cafer  combined  with  a  well-developed  nuchal  crescent, 
or  nearly  pure  auratus  with  the  red  malar  stripes  of  a  cafer. 


152  PROBLEMS  OF  GENETICS 

Sometimes  the  body  plumage  is  that  of  C.  auratus  with  the  head 
nearly  as  in  pure  cafer,  or  exactly  the  reverse  may  occur.  Or  we 
may  have  the  general  plumage  as  in  cafer  with  the  throat  and 
crown  as  in  auratus,  and  the  malar  stripe  either  red  or  black, 
or  mixed  red  and  black,  and  so  on  in  almost  endless  variations, 
it  being  rare  to  find,  even  in  birds  of  the  same  nest,  two  indi- 
viduals alike  in  all  their  features  of  colouration.  Usually  the 
first  trace  of  cafer  seen  in  auratus  manifests  itself  as  a  mixture  of 
red  in  the  black  malar  stripe,  either  as  a  few  red  feathers,  or  as  a 
tipping  of  the  black  feathers  with  red,  or  with  merely  the  basal 
portion  of  the  feathers  red.  Sometimes,  however,  there  is  a 
mixture  of  orange  or  reddish  quills,  while  the  malar  stripe  remains 
normal.  In  C.  cafer  the  traces  of  auratus  are  usually  shown  by  a 
tendency  to  an  incipient  nuchal  crescent,  represented  often  by 
merely  a  few  red-tipped  feathers  on  the  sides  of  the  nape;  at 
other  times  by  a  slight  mixture  of  black  in  the  red  malar  stripe." 
Such  a  state  of  things  accords  very  imperfectly  with  expecta- 
tions under  any  received  theory  of  Evolution.  As  in  some  of 
the  instances  discussed  in  the  first  chapter  we  have  here  two 
fairly  definite  forms,  nearly  allied,  which  on  any  evolutionary 
hypothesis  must  have  been  evolved  either  the  one  from  the  other, 
or  both  from  a  third  form  at  a  time  not  very  remote  from  the 
present,  as  time  must  be  measured  in  evolution.  Yet  though 
intermediates  exist  in  some  quantity,  no  one  can  for  a  moment 
suggest  that  they  are  that  definite  intermediate  from  which 
auratus  and  cafer  descend  in  common.  One  cannot  imagine  that 
the  immediate  ancestor  of  these  birds  was  a  mosaic,  made  up  of 
asymmetrical  patches  of  each  sort:  but  that  is  what  many  of  the 
intermediates  are.  It  is  not  much  easier  to  suppose  the  ancestor 
to  have  been  a  nondescript,  with  a  compromise  between  the 
developed  characters  of  each,  with  quills  buff,  malar  stripes 
neither  black  nor  red,  with  a  trace  of  nuchal  crescent,  and  so  on. 
Such  Frankenstein-monsters  have  played,  a  considerable  part  in 
the  imaginations  of  evolutionary  philosophers,  but  if  it  were 
true  that  there  was  once  a  population  of  these  monsters  capable 
of  successful  existence,  surely  they  should  now  be  found  as  a 
population  occupying  the  neutral  zone  between  the  two  modern 


OVERLAPPING  FORMS  153 

forms.  Yet,  though  much  remains  to  be  done  in  clearing  up  the 
facts,  one  thing  is  certain,  namely  that  the  neutral  zone  has  not  a 
definite  and  normally  intermediate  population,  but  on  the  con- 
trary it  is  peopled  by  fragments  of  the  two  definite  types  and 
miscellaneous  mongrels  between  them. 

On  the  other  hand,  one  cannot  readily  suppose  that  either 
form  was  the  parent  of  the  other.  The  process  must  have 
involved  both  addition  and  loss  of  factors,  for  whatever  hypoth- 
esis be  adopted,  such  changes  must  be  supposed  to  have  occurred. 
A  careful  statistical  tabulation  of  the  way  in  which  the  characters 
are  distributed  in  the  population  of  the  mixed  zone  would  be 
of  great  value,  and  till  that  has  been  done  there  is  little  that  can 
be  said  with  certainty  as  to  the  genetics  of  these  characters. 
In  the  collection  of  Dr.  Bishop  of  New  Haven  I  was  very  kindly 
allowed  to  examine  a  sample,  all  taken  at  random,  near  together, 
in  Saskatchewan.  There  were  females  4  adult,  2  young;  males 
4  adult  and  5  young.  This  number,  though  of  course  insufficient, 
is  enough  to  give  some  guide  as  to  the  degree  of  definiteness  which 
the  characters  generally  show  in  their  variations.  Of  the  15 
birds,  8  had  simply  yellow  quills;  2  had  red;  I  was  almost  red 
but  had  one  yellow  tail-quill;  3  were  intermediate  and  I  was 
buff.  As  regards  the  malar  patch,  which  can  only  be  determined 
properly  in  the  adult  males,  I  was  red,  I  was  approximately  red, 
2  intermediate.  As  to  nuchal  crescent  4  females  had  none,  2 
females  very  slight;  7  males  had  it,  I  had  only  a  slight  crescent, 
and  I  had  none.  In  point  of  quills  therefore  10  were  definite 
out  of  15;  in  point  of  crescent,  n  were  definite  out  of  15;  and  in 
point  of  malar  patch  I  only  was  definite  out  of  4.  The  last  is  a 
feature  directly  dependent  on  age  and  so  counts  for  less,  but  as 
regards  the  other  two  features  there  is  some  indication  that  the 
factors  show  definiteness  in  their  behaviour.  It  must  be  re- 
membered that  we  have  no  knowledge  what  the  heterozygous 
form  may  be,  and  in  the  case  of  red  and  yellow  it  is  probably  a 
reddish  buff.  The  patch-works  are  no  doubt  to  be  compared  with 
other  well-known  pied  forms,  and  in  these  we  must  suppose  the 
active  factor  broken  up,  which  it  probably  can  be  very  easily. 
The  asymmetry,  which  Allen  notices  as  so  marked  a  feature,  in  the 


154  PROBLEMS  OF  GENETICS 

distribution  of  the  red  and  yellow  quills  of  the  tail  especially, 
recalls  that  of  the  black  markings  in  the  pied  Canaries.  As  is  well 
known  to  students  of  variations  some  pigment-factors  in  some 
animals  are  apparently  uncontrolled  by  symmetry,  while  in  other 
specific  cases  symmetry  is  the  rule.  On  the  other  hand  the 
blackness  or  redness  of  the  malar  patches  is,  I  think,  as  a  rule 
nearly  symmetrical.  It  should  be  mentioned  that  two  of  Dr. 
Bishop's  young  birds  belonged  to  the  same  nest,  one  a  female 
with  red  quills,  the  other  a  male  with  yellow.  Both  are  without 
crescent. 

As  to  the  question  whether  certain  combinations  of  characters 
occur  with  special  frequency,  the  evidence  is  insufficient  to  give  a 
definite  answer.  Among  all  the  birds  I  have  seen  in  America 
or  in  England  I  have  not  yet  found  one  having  the  malar  patches 
black  without  any  nuchal  crescent.  Of  Dr.  Bishop's  8  adults 
not  one,  however,  showed  the  combination  of  the  three  chief 
features  normal  for  auratus  or  for  cafer. 

Besides  the  two  forms  that  we  have  hitherto  considered, 
several  other  local  types  exist,  and  these  throw  some  further 
light  on  the  problem.  Of  these  the  most  important  in  this 
connexion  is  chrysoides,  which  inhabits  the  whole  of  southern 
California  and  the  mainland  opposite.  This  remarkable  form 
is  as  Allen  says,  very  different  from  auratus  except  that  it  has 
the  quills  yellow  like  auratus,  not  red  like  cafer.  So  that  we 
find  here  in  the  extreme  west  of  the  whole  distribution  a  type 
agreeing  in  one  of  its  chief  features  with  the  eastern  type.  Be- 
tween this  and  cafer  intergrades  have,  according  to  Allen,  not 
been  found.  The  relations  of  this  chrysoides  are,  Allen  thinks, 
rather  with  mexicanoides ,  a  southern,  smaller  race  with  colours 
more  intense,  which  inhabits  Guatemala,  but  however  that  may 
be,  it  must  be  regarded  as  a  cafer  which  has  lost  its  red  quills. 
The  island  of  Guadeloupe  off  Lower  California  has  an  island 
form.  Beyond  the  other  side  of  the  continent  there  is  also  an 
island  form  of  auratus,  inhabiting  Cuba,  so  that  clearly  the  yellow 
quills  can  extend  into  the  tropics. 

The  above  account  is  in  many  respects  incomplete,  but  it 
suffices  to  give  an  outline  of  the  chief  facts.  The  whole  problem 


OVERLAPPING  FORMS  155 

is  complicated  by  the  undoubted  effects  of  an  uncertain  amount 
of  migration,  and  in  many,  perhaps  all,  districts,  the  winter 
population  differs  from  the  summer  population  of  the  same 
localities.  The  existence  of  these  seasonal  ebbs  and  flows  is 
now  well  known  to  ornithologists,  and  most  of  the  bird  species 
of  temperate  regions  are  subject  to  them. 

Difficult  as  it  may  be  to  conceive  the  actual  process  of  origin 
of  the  two  types  auratus  and  cafer,  it  is  I  think  still  harder  to 
suggest  any  possible  circumstance  which  can  have  determined 
their  development  as  distinct  races,  or  which  can  maintain 
that  distinctness  when  created.  Some  will  no  doubt  be  disposed 
to  appeal  once  more  to  our  ignorance  and  suggest  that  if  we  only 
knew  more  we  should  see  that  the  yellow  quills,  the  black 
"moustache"  and  the  red  crescent,  specially  qualify  auratus 
for  the  north  and  eastern  region,  and  the  red  quills,  red  "mou- 
stache" and  absence  of  crescent  fit  cafer  to  the  conditions  of 
its  homes.  Each  can  judge  for  himself,  but  my  own  view  is 
that  this  is  a  vain  delusion,  and  that  to  cherish  it  merely  blunts 
the  receptivity  of  the  mind,  which  if  unoccupied  with  such  fancies 
would  be  more  ready  to  perceive  the  truth  when  at  last  it  shall 
appear.  Think  of  the  range  of  conditions  prevailing  in  the 
country  occupied  by  auratus — a  triangle  with  its  apex  in  Florida 
and  its  base  the  whole  Arctic  region  of  North  America.  Is  it 
seriously  suggested  that  there  is  some  element  common  to  the 
"conditions"  of  such  an  area  which  demands  a  nuchal  crescent 
in  the  Flickers,  though  the  birds  of  the  cafer  area,  almost  equally 
varied,  can  dispense  with  the  same  character?  Curiously  enough, 
the  geographical  variation  of  Sphyropicus  varius,  another  though 
a  very  different  Woodpecker6  shows  that  conversely  the  nuchal 
crescent  can  be  dispensed  with  in  the  Eastern  form  though  it 
is  assumed  by  the  Western.7 

8  The  Sap-suckers  feed  on  trees  and  somewhat  resemble  our  Spotted  Wood- 
peckers in  general  appearance.  Colaptes  feeds  on  the  ground  and  corresponds 
perhaps  rather  with  the  European  Green  Woodpecker. 

7  For  an  introduction  to  this  example  I  am  indebted  to  Mr.  W.  D.  Miller  of 
the  American  Museum  of  Natural  History.  Some  account  of  the  facts  is  given  by 
Baird,  Brewer,  and  Ridgway  (A  Hist,  of  N.  Amer.  Birds,  1874,  II,  pp.  540,  544, 
etc.).  5.  -varius  occupies  the  whole  country  in  suitable  places  from  the  Atlantic 
to  the  eastern  slopes  of  the  Rockies,  and  all  Mexico  to  Guatemala.  5.  nuchalis  was 


156  PROBLEMS  OF  GENETICS 

Allen  points  out  the  interesting  additional  fact  that  super- 
posed upon  each  of  the  two  distinct  forms,  auratus  and  cafer, 
are  many  geographical  variations  which  can  very  naturally  be 
regarded  as  climatic.  Each  decreases  in  size  from  the  North 
southward,  as  so  many  species  do.8  They  become  paler  in  the 
arid  plains,  and  show  the  ordinary  phases  which  are  seen  in 
other  birds  having  the  same  distribution.  Such  differences 
we  may  well  suppose  to  be  determined  directly  or  indirectly,  by 
environment,  and  we  may  anticipate  with  fuller  knowledge  it 
will  be  possible  to  distinguish  variations  of  this  nature  as  in  the 
broad  sense  environmental,  from  the  larger  differences  separating 
the  two  main  types  of  Colaptes,  which  I  surmise  are  altogether 
independent  of  such  influences. 

It  is  generally  supposed  that  phenomena  like  those  now  so 
well  established  in  the  case  of  Colaptes  are  very  exceptional,  and 
as  has  already  been  stated  a  number  of  circumstances  must 
combine  in  order  that  they  may  be  produced.  I  suspect  however 
that  the  examples  are  more  numerous  than  is  commonly  thought. 
In  all  likelihood  the  three  forms  Sphyropicus  varius,  nuchalis 
and  ruber  are  in  a  very  similar  condition  though  the  details 
have  not,  so  far  as  I  know,  been  worked  out.  A  complex  example 
which  is  closely  parallel  to  the  case  of  Colaptes  was  described  by 
F  M.  Chapman9  at  the  same  date  as  Allen's  work.  This  is  the 
case  of  Quiscalus,  the  Crackles,  which  in  the  North  American 
Continent  have  three  fairly  distinct  forms  which  Chapman  speaks 
of  as  Q.  aeneus,  Q.  quiscula,  and  Q.  quiscula  aglaeus.  The  birds 
are  all,  so  far  as  pigment  is  concerned,  dark  blackish  brown,  but 
the  head  and  mantle  have  superposed  a  metallic  sheen  of  inter- 
ference-colours which  in  the  various  forms  take  different  tints, 

first  known  from  the  Southern  Rockies  only,  but  many  were  afterwards  taken  in 
Utah.  5.  ruber  is  restricted  to  the  Pacific  coast.  In  Ridgway's  opinion  all  three 
are  geographical  forms  of  one  species.  In  ruber  the  sexes  are  alike  having  both  a 
great  extension  of  the  red  in  the  throat,  and  a  red  crescent.  The  male  of  nuchalis 
grades  to  the  ruber  form,  but  the  female  does  not.  This  female  has  some  red  in 
the  throat  like  the  male  of  varius,  whereas  the  female  of  varius  has  a  whitish  throat. 

8  Not  only  vertebrates  but  the  marine  Crustacea  and  Mollusca  illustrate  this 
curious  "principle"  of  variation,  as  Canon  Norman  formerly  pointed  out  to  me 
with  abundant  illustrations.     There  are  of  course  cases  to  the  contrary  also. 

9  Chapman,  F.  M.,  Bull.  Amer.  Mus.,  IV,  1892,  p.  i;  see  also  Ridgway,  Birds 
of  North  and  Middle  America,  1902,  Part  II,  p.  214. 


OVERLAPPING  FORMS  157 

bluish  green,  bronze  green,  or  bronze  purple.  The  details  are 
complicated  and  difficult  to  appreciate  without  actual  specimens, 
but  the  two  common  types  are  sufficiently  distinct.  The  birds 
inhabit  the  whole  area  east  of  the  Rockies,  quiscula  aglaeus  oc- 
cupying Florida  and  the  Southern  States  southwest  of  a  band 
of  country  about  a  hundred  miles  broad  extending  roughly  from 
Connecticut  to  the  mouth  of  the  Mississippi;  and  aeneus  taking 
the  area  north  and  west  of  this  band.  In  discussing  this  case 
Chapman  expresses  the  same  view  as  Allen  does  in  the  Colaptes 
case,  that  there  are  two  distinct  populations,  substantially  fixed, 
and  that  the  band  of  country  in  which  they  meet  each  other 
has  a  mongrel  population,  with  no  consistent  type,  but  showing 
miscellaneous  combinations  of  the  character  of  the  two  chief 
types. 

The  warblers  of  the  genus  Helminthophila  provide  another 
illustration  which  has  points  of  special  interest.  The  two  chief 
species  are  H.  pinus,  which  has  a  yellow  mantle  and  lower  parts, 
white  bars  on  the  wings,  a  black  patch  behind  the  eyes  and  a 
broad  black  mark  on  the  throat ;  and  H.  chrysoptera  with  dark  grey 
mantle  and  pale  whitish  grey  lower  parts,  yellow  bars  on  the 
wings,  and  grey  marks  on  cheeks  and  throat  where  pinus  has 
black.  These  two  birds  are  exceeding  distinct,  and  in  addition 
their  songs  are  quite  unlike.  H.  pinus  ranges  through  the  eastern 
United  States  up  to  Connecticut  and  Iowa.  H.  chrysoptera  is  a 
northern  form  extending  down  to  Connecticut  and  New  Jersey. 
Both  are  migrants. 

In  these  two  States,  where  the  two  types  overlap,  certain 
forms  have  been  repeatedly  found  which  have  been  described  as 
two  distinct  species,  Lawrencei  and  leucobronchialis.  Dr.  L.  B. 
Bishop  and  Mr.  Brewster  showed  me  two  long  series  of  Hel- 
minthophila containing  various  intergrades  between  the  four 
named  kinds,  and  details  regarding  these  may  be  found  in 
Chapman's  North  American  Warblers  and  in  Dr.  Bishop's  paper 
in  Auk,  1905,  XXII.  Though  the  characters  evidently  break 
up  to  some  extent,  the  series  can  be  represented  as  due  to  re- 
combinations of  definite  factors  more  easily  than  the  others 
which  I  have  described.  The  differentiating  characters  are: 


158  PROBLEMS  OF  GENETICS 

Pinus  Chrysoptera 

1.  Mantle  and  lower  parts  yellow  (Y1).        i.  Mantle  and  lower  parts  grey  (y1). 

2.  Wing-bars  white  (y2).  2.  Wing-bars  yellow  (Y2). 

3.  Cheek  and  throat  not  black  (b).  3.  Cheek  and  throat  black  (B). 

The  grey  pigment  of  the  mantle  is  common  to  both,  but  is 
masked  by  the  yellow  in  pinus,  the  net  result  being  an  olive- 
green.10 

I  am  much  indebted  to  Dr.  F.  M.  Chapman  for  the  loan  of 
the  coloured  plate  in  which  these  distinctions  are  shown.  It  first 
appeared  in  his  book,  North  American  Warblers. 

We  cannot  tell  whether  yellow  or  not-yellow  is  due  to  the 
presence  of  a  factor,  but  we  may  suppose  that  one  or  other 
gives  the  special  colour  to  the  parts.  The  black  of  character  3 
is  no  doubt  a  dominant.  Thus  pinus  becomes  Yxy2b  and  chry- 
soptera  in  y1Y2B.  The  Lawrencei  which  has  the  underparts 
yellow,  wing-bars  white,  and  black  patches  is  Yxy2B  and  leuco- 
bronchialis  which  has  mantle  and  underparts  not-yellow,  wing- 
bars  yellow  and  no  black  patches  is  y1Y2b.  This  representation, 
it  should  be  clearly  understood,  is  tentative  and  approximate 
only.  The  characters  are  not  really  sharp,  for  there  is  much 
grading ;  but  allowing  for  the  effects  of  heterozygosis  and  for  some 
actual  breaking-up  of  factors  I  believe  it  gives  a  fairly  correct 
view  of  the  case.  In  particular  we  can  see  how  it  meets  the  dif- 
ficulty which  Chapman  felt  in  accepting  leucobronchialis  as  in 
any  sense  derived  from  pinus  which  has  a  yellow  breast,  and 
chrysoptera  which  has  a  black  throat,  seeing  that  leucobronchialis 
has  neither.  We  now  recognize  at  once  that  this  form  could  be 
produced  by  ordinary  re-combination  of  the  absence  of  Y1  with 
the  absence  of  B. 

I  note  also  with  great  interest  that  the  modern  observers 
agree  that  the  so-called  hybrids  may  have  the  song  either  of 
the  one  species,  or  of  the  other,  or  a  song  intermediate  between 
the  two.  It  may  also  be  added  that  these  two  types  have  several 

10  It  would  aid  greatly  in  factorial  analysis  if  the  descriptive  term  "green" 
could  be  avoided  in  application  to  cases  where  the  green  effect  is  due  only  to  a  mix- 
ture of  black  and  yellow  pigments.  The  absence  of  yellow  is  the  sole  difference 
between  the  mantle  and  underparts  of  pinus  and  chrysoptera. 


FIG.   i.  fltlminthophila  pinus,  male. 

FIG.   2.  fIdmi>:lJiophila  pinus,  female. 

FIG.  3.  "Lawrence's  Warbler,"  male;  one  of  the  integrading  forms. 

FIG.  4.  "Brewster's  Warbler,"  male;  another  of  the  intergrading  forms 

FIG.  5.  Helminthopkila  chrysoptera,  male-. 

FIG.  6.  Helminthophila  chrysoptera,  female. 


L 

if 


f.C  I/ 


158  PROBLEMS  OF  GENETICS 

Pinus  Chrysopiera 

z.  Mantle  and  lower  parts  yellow  (Y1).  z.  Mantle  and  lower  parts  grey  (y1). 

a.  Wing-bars  white  (y*).  2.  Wing-bars  yellow  (Y1). 

3.  Cheek  and  throat  not  black  (b).  3.  Cheek  and  throat  black  (B). 

The  grey  pigment  of  the  mantle  is  common  to  both,  but  is 
masked  by  the  yellow  in  pinus,  the  net  result  being  an  olive- 
green.10 

I  am  much  indebted  to  Dr.  F.  M.  Chapman  for  the  loan  of 
the  coloured  plate  in  which  these  distinctions  are  shown.  It  first 
appeared  in  his  book,  North  American  Warblers. 

We  cannot   tell  whether  yellow  or  not-yellow  is  due  to  the 

presence  of  a  factor,  but  we  may  suppose  that  one  or  other 

gives  the  special  colour  to  the  parts.     The  black  of  character  3 

•Moubt  a  dominant.     Thus  pinus  becomes  Yly2b  and  chry- 

The  Laivrencei  which   has,  the   underparts 

white,  and 

.anno*  anH>sis*m  srf*  to  one  ;9iBm  -.-wid-i 

i  arfi  to  liftoius  ;<A&m  ".-wld  -r:77  J 

.01^ 


.g   . 

.^osis  ana  for  some 

,d  .01*5 


airy  correct 

se.  In  particular  we  can  see  how  it  meets  the  dif- 
ficulty which  Chapman  felt  in  accepting  leucobronchialis  as  in 
any  sense  der  which  has  a  yellow  breast,  and 

chrysoptcr  throat,  seeing  that  leucobronchialis 

has  neither.  We  now  recognize  at  once  that  this  form  could  be 
produced  by  ordinary  re-combination  of  the  absence  of  Y1  with 
the  absence  of  B. 

I  note  also  with  great  interest  that  the  modern  observers 

that  the  so-called  hybrids  may  have  the  song  either  of 

the  one  species,  or  of  the  other,  or  a  song  intermediate  between 

the  two.     It  may  also  be  added  that  these  two  types  have  several 

would  aid  greatly  in  factorial  analysis  if  the  descriptive  term  "green" 
could  be  avoided  in  application  to  cases  where  the  green  effect  is  due  only  to  a  mix- 
ture of  black  and  yellow  pigments.     The  absence  of  yellow  is  the  sole  difference 
•*  en  the  mantle  and  underparts  of  pinus  and  chrysoptera. 


OVERLAPPING   FORMS  159 

times  been  seen,  in  the  breeding  season,  paired  with  each  other 
or  with  one  of  the  other  combinations. 

Allen11  has  described  another  excellent  American  example, 
the  Tits  of  the  group  Baeolophus  bicolor -atricristatus.  The  form 
bicolor  belongs  to  the  eastern  States  and  ranges  from  the  Atlantic 
coast  to  the  Great  Plains,  and  atricristatus,  of  east  Mexico,  ex- 
tends from  Vera  Cruz  to  central  Texas.  In  southern  and 
central  Texas  the  breeding  ranges  adjoin,  and  in  this  country 
various  intermediates  occur.  The  chief  types  differ  in  two  main 
points. 

B.  bicolor  B.  atricristatus 

Forehead  varies  from  deep  black  to  dull  Forehead  white  to  huffish  white. 

black,  suffused  with  rusty  brown. 

Crown  and  crest  grey,  slightly  darker  Crown  and  crest  black,  abruptly  con- 

than  the  black.  trasting  with  the  back. 

The  intergrades  between  the  two  have,  as  usual,  received  specific 
names.  A  detailed  description  is  given  by  Allen,  from  which 
it  appears  that  the  gradation  is  very  complete.  In  one  case  a 
series  of  16  adults  were  all  intermediates.  It  is  not  stated  whether 
the  collector  took  these  at  random,  but  from  the  local  lists  it  is 
clear  that  the  types  are  found  not  far  away  from  the  place  where 
the  intergrades  were  shot. 

Another  very  striking  case  is  that  of  the  Tanagers,  of  the 
genus  Rhamphocoelus.  In  this  group  there  are  several  local  forms 
which  are  related  to  each  other  in  remarkable  ways.  The  forms 
known  as  passerinii  and  icteronotus  exhibit  the  clearest  phenomena 
of  intergradation.  The  species  passerinii  has  a  brilliant  scarlet 
and  black  male,  and  it  inhabits  Honduras  and  Nicaragua. 
Proceeding  southwards  along  the  isthmus  we  find  next  costari- 
censis  which  has  a  male  like  that  of  passerinii  (but  a  female 
with  more  orange  than  the  olive-grey  female  of  passerinii). 
Next  we  come  to  Panama  which  is  occupied  by  icteronotus, 
sharply  distinguished  from  passerinii  by  the  fact  that  the  scarlet 
is  replaced  by  lemon-yellow.  This  same  icteronotus  occurs  again 
as  a  pure  type  in  Ecuador  and  many  other  parts  of  South  America; 
but  Colombia,  between  Panama  and  Ecuador,  contains  scarlets 
like  passerinii,  yellows  like  icteronotus,  and  various  intergrades 

11  Bull.  Amer.  Mus.  Nat.  Hist.,  XXIII,  1907,  P-  46?- 


160  PROBLEMS  OF  GENETICS 

of  several  shades  of  orange.  The  passerinii  males  from  Nica- 
ragua are  indistinguishable  from  those  of  Colombia,  and  the 
icteronotus  of  Ecuador  are  the  same  as  those  in  Panama.  The 
orange  intergrades,  doubtless  heterozygous  forms,  though  col- 
lected at  the  same  locality  (Medellin  in  Colombia)  as  several 
pure  yellows  and  pure  scarlets,  are  in  the  British  Museum  series 
sorted  out  as  a  separate  species  under  the  name  chrysonotus! 
Complications  are  introduced  by  the  relations  of  these  forms  to 
another  named  type,  flammigerus,  but  we  may  for  our  purpose 
leave  that  out  of  consideration,  and  say  that  the  order  of  geo- 
graphical sequence  from  Honduras  to  Ecuador  is  (i)  scarlet, 
(2)  yellow,  (3)  mixture  of  types,  scarlet,  yellow,  orange,  (4) 
yellow. 

Similar  examples  exist  in  the  birds  of  the  old  world,  but  I  do 
not  know  of  any  that  have  been  studied  so  fully  as  those  of 
America.  The  best  known  is  that  of  the  two  Rollers,  Coracias 
indicus  which  spreads  from  Asia  Minor  through  Persia,  Balu- 
chistan, the  Indian  Peninsula  and  Ceylon,  and  affinis  which 
ranges  from  Nepal,  through  Assam,  Tenasserim  and  the  Indo- 
Chinese  countries.  The  two  types  are  very  different  and  may  be 
distinguished  as  follows: 

C.  indicus  C.  affinis 

Mantle  drab  brown-chestnut.  Dark  olive-green. 

Breast  chestnut.  Dull  purple  brown. 

Throat  purplish,  streaked  with  white.  Purple,  streaked  with  blue. 

Upper  tail-coverts  indigo.  Turquoise. 

The  wings  are  the  same  in  both.  In  the  provinces  of  Nepal, 
Sikhim,  and  Darjiling  the  two  species  coexist,  with  the  result 
that  intergrades  have  been  frequently  recorded.  The  line  of 
intergradation  extends  to  the  coast,  and  birds  showing  various 
combinations  of  the  two  types  from  the  Calcutta  district  exist 
in  collections.12  The  case  is  interesting  inasmuch  as  like  that  of 
Quiscalus  it  shows  a  series  of  combinations  of  various  metallic 
colours.  Some  of  these  are  probably  evoked  by  the  development 
of  pigment  behind  striations  or  other  interferences  already  exist- 
ing, but  in  the  present  state  of  knowledge  it  would  be  quite  im- 

12  References  on  this  subject  will  be  found  in  Brit.  Mus.  Cat.  Birds,  XVII,  p.  13. 


OVERLAPPING  FORMS  161 

possible  to  suggest  what  the  actual  factors  producing  these  ap- 
pearances may  be. 

There  are,  naturally,  many  other  cases  among  birds  which 
are  suspected  of  being  in  reality  comparable,  but  in  most  of  them 
the  evidence  is  still  inadequate.  Among  Lepidoptera  also  there 
are  a  few  of  these ;  perhaps  the  most  striking  is  that  of  Basilarchia 
"proserpina."1*  The  genus  is  well  known  to  European  col- 
lectors under  the  name  LimenitiSj  of  which  we  in  England  have 
one  species,  L.  sibylla,  the  "White  Admiral."  A  species  very 
like  sibylla  in  general  appearance  is  common  in  the  northern 
parts  of  the  United  States,  ranging  through  Canada  and  Northern 
New  England,  but  rarely  south  of  Boston.  This  species  has 
the  conspicuous  white  bands  across  both  wings  like  our  sibylla. 

There  is  also  a  more  Southern  type  known  as  astyanax, 
which  is  very  different  in  its  appearance,  being  without  the  white 
bands  and  having  a  broad  irroration  of  blue  scales  on  the  posterior 
border  of  the  hind  wings.  The  two  are  so  distinct  that  one  would 
not  be  tempted  to  suspect  any  very  close  relation  between  them. 
In  its  distribution  astyanax  is  described  by  Field  as  replacing 
arthemis  south  of  latitude  42°.  About  Boston  it  is  much  more 
common  than  arthemis. 

The  two  forms  encroach  but  little  on  each  other's  territory, 
but  where  they  do  coexist,  a  third  form,  known  as  proserpina,  is 
found  which  is  almost  intermediate,  with  the  white  bands  much  re- 
duced. There  is  now  no  doubt  that  this  proserpina  is  a  hetero- 
zygous form,  resulting  from  a  combination  of  the  characters  of 
arthemis  and  astyanax.  Field  succeeded  in  rearing  a  brood  of  16 
from  a  proserpina  mother  caught  wild  which  laid  31  eggs,  and  of 
these,  nine  (five  males,  four  females)  resembled  the  mother, 
being  proserpina,  and  seven  (four  males,  three  females)  were 
arthemis.  There  can  be  no  question  therefore  that  the  mother 
had  been  fertilised  by  a  male  arthemis  and  that  no-white-band 
is  a  factor  partially  dominant  over  the  white  band.  Another 
point  of  interest  which  Field  observed  was  that  the  proserpina 
female  refused  to  lay  on  birch,  poplar  or  willow,  but  accepted 

13  For  these  facts  I  am  indebted  to  Mr.  W.  L.  W.  Field,  who  has  lately  published 
an  account  of  his  observations  and  experiments.     See  especially,  Psyche, 
XVII,  No.  3,  where  full  references  to  previous  publications  are  given. 

12 


162  PROBLEMS  OF  GENETICS 

wild  cherry  (Prunus  serotina)  a  species  on  which  astyanax  can 
live,  though  that  tree  is  not  known  to  be  eaten  by  arthemis. 
Incidentally  also  the  observations  show  that  sterility  cannot  be 
supposed  to  be  the  bar  which  maintains  the  distinctness  of 
arthemis  and  astyanax. 

In  this  connection  Papilio  oregonia  and  bairdii  should  be 
mentioned.14  P.  oregonia  is  one  of  the  numerous  forms  like 
machaon,  but  rather  paler.  It  is  a  northern  insect,  inhabiting 
British  Colombia  east  of  the  Cascade  Range,  and  reaching  to 
Colorado.  P.  bairdii  is  a  much  darker  butterfly,  representing 
the  asterias  group  of  the  genus  Papilio.  Like  asterias  it  has  the 
abdomen  spotted  at  the  sides,  not  banded  as  in  the  machaon 
group.  It  belongs  to  Arizona  and  Utah  extending  into  Colorado. 
From  Colorado  the  form  brucei  is  described,  more  or  less  inter- 
mediate, like  bairdii  but  with  the  abdomen  banded  as  in  oregonia* 
W.  H.  Edwards  records  the  results  of  rearing  the  offspring  of  the 
bairdii-\ike  and  of  the  oregonia-\ike  mothers.  Each  was  found 
able  to  have  offspring  of  both  kinds,  that  is  to  say,  bairdii 
females  gave  both  forms,  and  oregonia  females  gave  both  forms. 
It  is  not  possible  to  say  which  is  dominant,  since  the  fathers  were 
unknown.  On  general  grounds  one  may  expect  that  the  bairdii 
form  will  be  found  to  dominate,  but  this  is  quite  doubtful. 

From  this  particular  discussion  I  omit  reference  to  those 
examples  in  which  the  permanently  established  types  are  ob- 
viously associated  with  special  conditions  of  life.  Where  con- 
siderable climatic  differences  exist  between  localities,  or  when  we 
pass  from  South  to  North,  or  from  the  plains  into  Alpine  levels 
we  often  find  that  in  correspondence  with  the  change  of  climate 
there  is  a  change  in  the  characteristics  of  a  species  common  to 
both.  When  I  say  "species "  in  such  a  connection  I  am  obviously 
using  the  term  in  the  inclusive  sense.  Some  would  prefer  to 
say  that  in  the  two  sets  of  conditions  two  representative  species 
exist.  Whichever  expression  be  preferred  it  is  plain  that  such 
examples  present  another  phase  of  the  problem  we  have  been 
just  considering,  and  in  them  also  we  have  an  opportunity  of 

14  For  the  facts  and  further  references  see  W.  H.  Edwards,  Butterflies  of  N. 
America,  2d  series,  Papilio  VII  and  X;  3d  series,  1897,  Papilio  IV,  Can.  Entom.t 
1895,  XXVII,  p.  239. 


OVERLAPPING  FORMS  163 

observing  the  consequences  of  the  overlap  of  two  closely  related 
types,  but  there  are  advantages  in  considering  them  separately. 
In  the  examples  hitherto  given,  with  the  possible  exception  of 
the  Papilios,15  the  two  fixed  types  severally  range  over  so  ex- 
tensive a  region  that  it  may  fairly  be  supposed  that  in  the  dif- 
ferent parts  they  are  subject  to  considerable  diversities  of  climate. 
There  is  no  outstanding  difference  that  we  know  distinguishing 
the  habitats  of  the  two  forms;  but  in  comparing  Alpine  with 
Lowland  forms,  or  essentially  northern  with  essentially  southern 
forms  we  do  know  an  external  circumstance,  temperature,  that 
may  reasonably  be  supposed  to  have  an  influence,  direct  or  indi- 
rect, on  the  population. 

15 1  think  this  case  is  fairly  included  because  the  machaon  type  is  so  widespread 
that  it  cannot  be  regarded  as  a  product  of  a  Northern  climate,  nor  can  asterias  be 
claimed  as  especially  a  warm  country  form,  seeing  that  brevicauda,  which  is  scarcely 
distinguishable  from  asterias,  inhabits  Newfoundland  (having  a  curious  phase 
there  in  which  the  yellow  is  largely  replaced  by  red). 


CHAPTER  VIII 

LOCALLY  DIFFERENTIATED  FORMS.     Continued 

CLIMATIC  VARIETIES 

In  this  chapter  we  will  examine  certain  cases  which  illustrate 
phenomena  comparable  with  those  just  considered,  though  as 
I  have  already  indicated,  they  form  to  some  extent  a  special 
group.  The  outstanding  fact  that  emerges  prominently  from 
the  study  of  the  local  forms  is  that  when  two  definite  types, 
nearly  allied,  and  capable  of  interbreeding  with  production  of 
fertile  offspring,  meet  together  in  the  region  where  their  dis- 
tributions overlap,  though  intergrades  are  habitually  found, 
there  is  no  normally  or  uniformly  intermediate  population  oc- 
cupying the  area  of  intergradation.  Such  phenomena  as  these 
must,  I  think,  be  admitted  to  have  great  weight  in  any  attempt 
to  construct  a  theory  of  evolution.  True  we  must  hesitate  in 
asserting  their  positive  significance,  but  I  see  no  escape  from  the 
conclusion  that  they  throw  grave  doubt  on  conventional  views. 
Again  and  again  the  same  question  presents  itself.  If  A  and  B 
lately  emerged  from  a  common  form  why  is  that  common  form 
so  utterly  lost  that  it  does  not  even  maintain  itself  in  the  region 
of  overlapping?  Almost  equally  difficult  is  it,  in  the  cases  which 
I  have  numerated,  to  apply  concrete  suggestions  based  on  any 
factorial  scheme.  We  may  see  that  in  Heliconius  erato  the  type 
with  the  red  mark  on  the  hind  wing  probably  contains  a  dominant 
factor,  and  that  where  the  red  mark  is  absent  the  metallic  colours 
are  exposed ;  and  that  similarly  the  green  metallic  colour  may  have 
another  factor  which  distinguishes  it  from  the  blue.  In  this  way 
we  can  fairly  easily  represent  the  various  types  of  erato  on  a 
factorial  system  as  the  result  of  the  various  possible  combinations 
of  two  pairs  of  factors.  But  there  we  stop,  and  we  are  quite 
unable  to  suggest  any  reason  why  one  area  should  have  the  red 
and  the  green  type  while  another  should  have  the  blue  also.  So 
again  with  Colaptes  or  the  Warblers.  By  application  of  a  fac- 

164 


CLIMATIC  VARIETIES  165 

torial  system,  admittedly  in  a  somewhat  lax  fashion,  the  genetic 
interrelations  of  the  types  can  be  represented;  but  how  it  comes 
about  that  each  type  maintains  a  high  degree  of  integrity  in 
its  own  region  we  can  only  imagine.  Each  has  in  actual  fact 
a  stability  which  the  intermediate  forms  have  not,  but  we  cannot 
yet  analyse  the  nature  of  that  stability.  Mendelian  conceptions 
show  us  how  by  segregation  the  integrity  of  the  factors  can  be  in 
some  degree  maintained,  but  not  why  certain  combinations  of 
factors  should  be  exceptionally  stable.  All  that  is  left  us  to 
fall  back  on  is  the  old  unsatisfying  suggestions  that  some  com- 
binations may  have  greater  viability  than  others,  that  there 
may  be  a  tendency  for  like  to  mate  with  like,  and  so  forth. 

These  difficulties  acquire  more  than  ordinary  force  in  those 
cases  in  which  the  two  fixed  types  inhabit  regions  differing  in 
some  respect  so  obvious  and  definite  that  we  are  compelled  to 
regard  each  type  as  climatic  and  as  specially  adapted  to  the 
conditions.  When  for  example  an  animal  has  a  distinct  type 
never  met  with  except  in  Arctic  or  Alpine  conditions,  and  another 
type  proper  to  the  plains  and  temperate  regions,  what  are  the 
characteristics  of  the  population  of  intermediate  latitudes  or 
at  intermediate  levels?  Some  of  the  examples  discussed  in  the 
last  chapter  may  be  instances  of  this  very  nature,  but  even  if 
they  are  not,  others  are  forthcoming  which  certainly  are.  The 
evidence  of  these  cases  leads  to  the  suspicion  that  with  further 
knowledge  they  will  be  found  to  consist  of  two  classes,  some  in 
which  the  observer  as  he  passes  from  the  one  climate  to  the  other 
will  find  the  intermediate  area  actually  occupied  by  a  population 
of  intermediate  character,  and  others  in  which,  though  we  may 
presume  the  maintenance  of  intermediate  conditions  in  the  tran- 
sitional area,  there  is  no  definite  transitional  population.  This 
interrupted  or  discontinuous  distribution  seems,  so  far  as  I 
have  means  of  judging,  to  be  by  far  the  more  common  of  the  two. 
I  do  not  doubt  that  by  sufficient  search  individuals  representing 
every  or  almost  every  transitional  form  can  be  found,  but  it  is 
apparently  rare  that  populations  corresponding  to  these  several 
grades  can  be  seen.  The  question  has  in  few  if  any  cases  been 
studied  with  precision  sufficient  to  provide  a  positive  answer; 


166  PROBLEMS  OF  GENETICS 

but  I  suspect  that  real  and  complete  continuity,  in  the  sense 
thus  denned,  will  only  be  found  where  the  character  of  the  local 
populations  depends  directly  on  the  conditions  of  life,  and  shows 
an  immediate  response  to  changes  in  them  apart  from  that  post- 
poned response  which  we  suppose  to  be  achieved  by  selection. 
Obviously  the  character  must  be  one,  like  size  for  instance, 
capable  of  sensibly  complete  gradation. 

The  only  example  I  have  met  with  of  the  phenomenon  of 
anything  like  a  complete  intergradation  between  local  types 
really  distinct  in  kind  is  that  provided  by  the  butterfly  Pararge 
egeria.  It  is  well  known  to  entomologists  that  this  insect  exists 
in  two  very  different  types,  a  northern  one,  the  "Speckled  Wood " 
of  England,  in  which  the  spots  are  a  pale  whitish  yellow,  and  a 
southern  type  having  the  full  fulvous  colour  that  we  know  as 
characteristic  of  megaera,  the  "Gatekeeper."  It  appears  that 
Linnaeus  gave  the  name  egeria  to  the  southern  type,1  and  our 
own  is  now  called  egerides.  Broadly  speaking,  so  far  as  Great 
Britain,  France,  and  the  Spanish  Peninsula  are  concerned,  the 
tawny-coloured  egeria  occupies  Spain  and  western  France  up 
to  the  latitude  of  Poitiers  and  the  pale  yellow  egerides  extends  from 
Scotland,  where  it  has  a  scanty  distribution,  through  southern 
England,  where  in  suitable  localities  it  is  common,  and  the  north 
of  France  to  Paris.2  The  two  types  when  placed  side  by  side 
are  strikingly  different  from  each  other,  and  are  an  excellent 
illustration  of  what  is  meant  by  climatic  variation.  The  insect 
is  not  a  great  traveller  and  probably  scarcely  ever  wanders  far 
from  its  home.  It  should  therefore  be  possible  by  collecting 
from  north  to  south  to  find  out  how  the  transition  is  effected, 
whether  suddenly  or  gradually.  This  at  various  times  I  have 
endeavoured  to  do,  but  I  am  still  without  exact  information  as 
to  the  population  in  certain  critical  areas.  In  addition  to  the 
information  derived  from  specimens  which  I  have  collected  or 
seen  in  the  collections  of  others  there  is  a  good  account  of  the 
general  distribution  in  Europe  given  by  the  Speyers,3  who  evi- 

1  Often  referred  to  by  older  writers  as  Meone,  Esper's  name. 

2  There  are  also  two  distinct  island  forms,  unlike  the  European,  Xiphia  of 
Madeira,  and  a  smaller  variety,  Xiphioides  of  Canary.      See  especially,  Baker, 
G.  T.,  Trans.  Ent.  Soc.  London,  1891,  p.  292. 

8  Speyer,  Adolf,  and  August.     Verbreitung  der  Schmetterlinge,  1858,  I,  p.  217. 


CLIMATIC  VARIETIES  167 

dently  paid  more  attention  to  the  subject  than  most  lepidopterists 
have  done,  and  many  more  recent  records.  In  particular 
Oberthiir4  has  published  many  details  as  to  the  distribution  in 
western  France  and  I  am  especially  indebted  to  Mr.  H.  Rowland- 
Brown  for  a  long  series  of  notes  as  to  the  distribution  in  France 
generally,  and  to  Mr.  H.  E.  Page  and  Dr.  T.  A.  Chapman,  Mr. 
Oberthur  Prof.  Arrigoni  degli  Oddi,  Mr.  H.  Williams  and  other 
correspondents,  for  showing  me  forms  from  many  localities.  The 
butterfly  is  attached  for  the  most  part  to  woods  of  deciduous  trees 
and  to  country  abounding  in  tall  hedges  or  rough  scrub.  It  is  not 
usually  to  be  found  in  highly  cultivated  districts  or  in  very  dry 
regions.  Hence  there  is  necessarily  some  want  of  continuity  in 
the  distribution  at  the  present  time  and  I  should  think  a  mile  or 
two  of  arable  land  without  big  hedges  would  constitute  a  barrier 
hardly  ever  passed.  The  larva  feeds  on  several  coarse  grasses, 
especially  Dactylis  glomerata.  Barrett  mentions  also  Triticum 
repens.  In  this  country  the  winter  is  usually  passed  in  the  larval 
stage,  but  I  have  found  that  in  captivity,  at  least,  there  is  much 
irregularity.  The  larvae  feed  whenever  the  weather  is  not  very 
cold  and  may  pupate,  but  if  sharp  cold  comes  on  when  they  are  pu- 
pating or  nearly  full-grown  they  often  get  killed  unless  protected. 

Some  writers  speak  of  a  difference  between  the  early  and 
later  broods,  but  I  have  never  noticed  this,  and  I  do  not  think 
that  the  general  tone  of  the  yellow  is  affected  by  the  seasons 
(see  Tutt,  Ent.  Rec.,  IX,  1897,  p.  37).5 

Beginning  at  the  south  of  Spain  the  thoroughly  fulvous  type 
egeria  is  common  at  Gibraltar  in  the  Cork  woods,  at  Granada, 
and  doubtless  generally.  Lederer  is  said  to  have  found  only 
this  type  in  Spain  (Speyer),  and  though  I  have  no  precise  in- 
formation as  to  other  places  in  the  Peninsula  north  of  Jaen  I  feel 
tolerably  sure  that  there  is  no  change  from  south  to  north.8 

4  Lepid.  Compare,  fsc.  Ill,  p.  372. 

6  Mr.  Rowland-Brown  has  called  my  attention  to  a  statement  by  Dr.  Vaillantin 
(Petites  Nouv.  Enl.,  II,  235)  that  in  Indre-et-Cher  the  first  brood  is  of  the  northern 
type  and  the  second  of  the  southern.  My  experience  is  that  in  captivity  these 
distinctions  do  not  occur,  and  I  have  true  egeria  as  first  brood  from  Vienne  and  as 
the  late  brood  from  the  Landes.  I  never  collected  in  Indre-et-Cher. 

6 1  have  since  seen  true  egeria  from  Ferrol  in  the  extreme  northwest,  which  was 
in  Mr.  Tutt  s  collection. 


168  PROBLEMS  OF  GENETICS 

Immediately  north  of  the  Pyrenees  we  still  meet  egeria  exclusively, 
and  up  to  Poitiers  at  least  there  is  no  noticeable  change.  But 
somewhere  between  Poitiers  and  the  bottom  of  the  Loire  valley 
at  Tours,  the  genuine  southern  type  comes  to  an  end,  and  the 
whole  population  begins  at  the  Loire  to  be  of  an  intermediate 
type,  easy  to  distinguish  both  from  egeria  and  from  egerides.  As 
to  the  exact  condition  of  the  species  in  the  fifty  miles  separating 
St.  Savin  on  the  Vienne  from  places  on  the  Loire  I  have  no  ade- 
quate information.  I  have  only  one  small  sample  from  there, 
but  it  does  contain  insects  both  of  the  southern  and  intermediate 
types  taken  on  the  same  day,  in  a  wood  near  Preuilly.  Oberthur 
also  states  that  at  Nantes  the  true  southern  form  exists  in  com- 
pany with  the  northern.  From  this  I  infer  that  the  southern 
form  extends  up  the  coast  further  than  it  does  inland,  but  I 
imagine  the  representative  spoken  of  as  northern  would  be  of 
usual  Brittany  or  intermediate  type. 

The  Vienne  river  joins  the  Loire,  so  the  true  southern  type 
reaches  over  into  the  basin  of  the  Loire.  From  the  Loire  (Tours, 
Corm6ry)  north  to  Calvados  (Balleroy)  only  the  intermediate 
is  found,  so  far  as  I  know,  and  the  same  type  extends  over 
Brittany.7  In  general,  however,  the  woods  near  Paris  have  the 
thoroughly  northern  type  egerides,  but  at  St.  Germain-en-Laye 
and  at  Etampes  (Oberthur)  the  population  approaches  the 
intermediate  type. 

On  the  whole  the  intermediate  type  is  certainly  less  homo- 
geneous than  either  of  the  extremes,  and  females  with  the  two 
central  spots  either  paler  or  more  fulvous  than  the  rest  are  not 
uncommon,  but  I  have  never  taken  one  on  the  Loire  or  in  Brit- 
tany which  I  should  class  with  either  of  the  extreme  types. 

Before  speaking  of  the  distribution  in  other  parts  of  France 
and  in  Europe  generally  I  will  briefly  state  the  results  of  my  breed- 
ing experiments.  The  work  was  done  many  years  ago  before 
we  had  the  Mendelian  clue,  and  it  is  greatly  to  be  hoped  that 
some  one  will  find  opportunities  of  repeating  it.  Crossing  the 
English  and  the  thoroughly  southern  type  the  families  produced 

7  Mr.  G.  Wheeler  kindly  showed  me  a  series  identical  with  this  type,  from 
Guernsey,  and  others  from  near  Laon. 


CLIMATIC  VARIETIES  169 

agree  entirely  with  the  intermediates  of  Brittany  and  the  Loire. 
Reciprocals  are  alike.  Of  F2  I  only  succeeded  in  raising  very 
few  and  of  those  that  I  had  (about  30)  nearly  all  were  intermediate 
in  character,  though  perhaps  rather  less  uniform  than  FI.  One 
family  alone,  containing  only  4  specimens,  had  one  egerides, 
and  three  fulvous  intermediates.  As  the  case  stands  alone  I 
hesitate  whether  or  not  to  suppose  it  due  to  some  mistake.  More- 
over from  FI  crossed  back  with  the  respective  parental  types  I 
had  fairly  long  series,  especially  from  FI  X  the  southern  type,  and 
looking  at  these  families  I  cannot  see  any  clear  evidence  of  segre- 
gation. On  the  contrary,  I  think  that  though  there  are  slight 
irregularities,  they  would,  taken  as  a  whole,  be  classed  as  coming 
between  the  intermediate  type  and  the  extreme  form  used  as  the 
second  parent.  This  at  least  is  true  when  the  second  parent  was 
of  the  southern  type. 

On  this  evidence  I  have  regarded  the  case  as  one  in  which 
there  is  no  good  evidence  of  segregation  and  as  conforming  most 
nearly  with  the  conventional  view  of  gradual  transition  in  re- 
sponse to  climatic  influences.  Such  influence  must  however  be 
indirect;  for  I  reared  five  generations  of  the  northern  type  in 
England,  and  these,  though  they  included  several  abnormal- 
looking  specimens  in  the  last  generation  and  then  died  out,  did 
not  show  any  noticeable  change  from  the  fulvous  colour  of  the 
wild  type.  Merrifield8  also  found  that  heat  applied  to  pupae 
of  the  northern  type  produced  no  approach  to  the  southern 
type. 

Looking  at  the  facts  now  in  the  light  of  more  experience  it 
seems  to  me  just  possible  that  the  case  may  be  one  in  which,  as 
in  Nilson-Ehle's  Wheats,  the  dominant  differs  from  the  re- 
cessive in  having  two  pairs  of  factors  with  similar  effects.  The 
fulvous  type  for  example  may  have  two  or  more  elements  in 
separate  pairs  which  together  produce  the  full  effect,  and  the 
intermediate  may  have  one  of  these.  If  this  were  so,  some 
segregation  should  of  course  eventually  be  observable,  but  the 
proportion  of  the  various  fulvous  and  fulvous-intermediate 
individuals  would  be  large,  and  the  reappearance  of  actual  repre- 

8  Ent.  Rec.,  V,  1894,  p.  134. 


170  PROBLEMS  OF  GENETICS 

sentatives  of  the  northern  type  might  be  rare.  I  admit  that  this 
is  a  somewhat  strained  interpretation  of  the  facts,  and  as  yet  it 
is  not  entitled  to  serious  consideration.  Nevertheless  I  am  led 
to  form  some  such  expectation  partly  from  the  great  difficulty 
in  the  way  of  any  other,  partly  from  the  evidence  of  the  small 
mixed  sample  found  at  Preuilly  and  partly  from  the  statements 
given  by  Oberthiir.  There  are  moreover  other  features  in  the 
general  distribution  of  the  species  which  make  it  improbable 
that  the  dependence  on  climate  can  after  all  be  so  close.  Pub- 
lished lists  are  unfortunately  of  little  use  in  deciding  which  form 
occurs  at  a  particular  place,  because,  since  the  name  Meone  has 
ceased  to  be  used  for  the  southern  form,  there  is  no  complete 
unanimity  among  authors  as  to  the  application  of  the  names 
egeria  and  egerides,  and  unless  more  particulars  are  given,  either 
name  may  be  used  for  either  form.  Besides  this,  difficulty  arises 
from  the  fact  that  the  intermediate  type  is  not  generally  dis- 
tinguished at  all,  and  English  collectors  finding  it,  may  easily 
record  it  as  the  southern  type.  From  Staudinger's  note  on  the 
distribution,  I  gather  that  he,  on  the  contrary,  reckoned  the 
intermediate  with  the  northern  type,  as  do  the  Speyers  also. 
The  late  Mr.  J.  W.  Tutt  was  careful  to  distinguish  the  three 
forms  and  has  left  several  useful  records.  Easy  therefore  as  it 
might  seem  to  be  to  make  out  the  distribution  of  such  a  familiar 
insect  in  its  various  modifications,  there  are  serious  practical 
difficulties,  and  until  long  series  are  brought  together  with  this 
special  object  in  view  many  obscurities  will  remain. 

With  only  the  series  from  England,  the  west  of  France,  and 
Spain  before  one  it  would  be  easy  to  regard  the  successive  series 
of  tones  as  a  fair  measure  of  climate ;  the  brighter  the  colour,  the 
hotter  might  one  expect  the  locality  to  be.  Such  rough  corre- 
spondence is  often  to  be  observed  in  butterflies  and  birds.  It 
becomes  impossible  to  take  these  simple  views  in  the  light  of 
more  complete  knowledge.  Beginning  with  France  the  fulvous 
egeria  occupies  the  lower  valley  of  the  Rhone,  probably  from  well 
above  Lyon,  though  I  have  no  exact  information  respecting  the 
country  above  Avignon.  According  to  Speyer  it  also  takes  the 
department  of  Lozere.  The  same  authority  says  that  Puy-de- 


CLIMATIC  VARIETIES  171 

Dome  has  "egeria"  meaning  perhaps  the  intermediate  form, 
with  the  fulvous  form  much  less  commonly.  Next  comes  the 
curious  fact  that  though  the  Lower  Rhone  (Avignon,  Tarascon, 
Nimes)  has  the  true  fulvous  form,  Hyeres,  Cannes,  Grasse,  Nice, 
Digne,  and  Alassio  have  the  intermediate.  Savoy  has  the  inter- 
mediate (Chambery)  and  even  egerides  perhaps,  though  in  the 
same  latitude  on  the  west  of  France  there  is  nothing  but  the 
fulvous  type.  At  Chalseul  and  Besangon  (Doubs)  the  ordinary 
northern  type  is  found.  Switzerland  generally,  I  believe,  has  the 
northern  type,  but  Staudinger  gives  egeria  for  Valais  and  the 
intermediate  occurs  in  Vaud.9  The  south  side  of  the  Alps  has 
probably  colonies  of  the  pale  egerides,  and  of  intermediates. 
Orta,  with  a  very  hot  summer,  has  the  English  type  (Tutt,  Ent. 
Rec.,  XII,  1900,  p.  328).  Locarno  has  the  intermediate  (ibid., 
XV,  1903,  p.  321).  North  Italy  in  general  and  western  Pied- 
mont have  the  intermediate;  but  further  south  egeria  begins, 
at  what  region  I  do  not  know.  Speyer  gives  on  his  own  authority 
the  remarkable  statement  that  at  Florence  both  extremes  occur, 
but  chiefly  intermediates  between  the  two.  Mr.  R.  Verity 
however  kindly  informs  me  that  in  his  experience  this  is  not  so, 
and  that  neither  the  real  southern  type  nor  the  northern  occur 
there.  Sardinia,  Sicily,  Crete  all  have  the  southern  type. 
Greece  probably  has  various  types.  Staudinger  (Hor.  Ross.,  VII, 
1870,  p.  78)  says  intermediates  resembling  Nice  types  common 
everywhere,  but  from  "Greece"  the  British  Museum  has  a  series 
that  would  pass  for  English  specimens;  and  the  same  type  occurs 
near  Constantinople.  The  island  of  Corfu  has  a  pale  inter- 
mediate, distinct  from  egerides  but  approaching  it.  In  Roumania 
all  three  forms  are  recorded  from  various  places :  egeria  in  the 
Dobrutscha;  not  quite  typical  (presumably  an  intermediate) 
at  Bukharest;  intermediate  in  various  mountainous  localities 
as  well  as  in  Macedonia  and  Dalmatia;  but  egerides  in  Azuga 
at  about  3,000  feet.10  Hungary  has  the  true  egerides  also. 
(Cf .  Caradja,  Deut.  Ent.  Z/.,  IX,  p.  58.)  Mathew  records  the  same 

9  Mr.  Wheeler  has  some  pale  but  rather  worn  specimens  from  the  Rhone  Valley 
at  Vernayaz. 

10  See  Fleck,  E.,  Die  Macrolep.  Rumaniens,  Bui.  Soc.  Sciinte,  VIII,  1899, 
p.  720. 


172  PROBLEMS  OF  GENETICS 

from  Gallipoli  (E.  M.  M.,  1881,  p.  95).  Staudinger  does  not 
distinguish  the  intermediates  from  the  northern,  but  he  gives 
"egerides"  for  Armenia  and  Fergana  (Central  Asia).  As  against 
the  mere  proximity  of  a  great  mountain  chain  being  the  influence 
which  keeps  the  Riviera  population  intermediate  may  be 
mentioned  the  fact  that  the  northern  foothills  of  the  Pyrenees 
have  the  pure  southern  type,  and  the  climate  of  Cambo  must 
surely  be  far  cooler  than  that  of  Nice.  The  exact  locality  of 
the  Greek  specimens  is  not  given,  but  there  can  be  no  part  of 
Greece  which  is  not  much  hotter  in  summer  than  Brittany,  or 
Calvados,  which  have  the  intermediate,  not  the  English  type. 
In  face  of  these  facts  it  can  scarcely  be  maintained  that 
average  temperature  is  the  efficient  cause  of  the  particular  tone 
of  colour  which  the  butterfly  shows 'in  a  given  region.  Never- 
theless it  is  clear  that  climate  counts  for  much  in  determining 
the  distribution.  It  is  noticeable  that  though  the  pale  egerides 
can  be  established  in  a  warm  climate  we  never  find  egeria  in  cold 
climates,  and  even  the  intermediate  is  not  found  in  places  that 
have  a  hard  winter.  I  suspect  that  the  distribution  of  the 
broods  through  the  year  and  the  condition  of  the  animal  at  the 
onset  of  hard  frost  are  features  which  really  determine  whether 
a  strain  can  live  in  a  particular  place  or  not.  Though  the  truth 
of  the  suggestion  cannot  be  tested  by  experiments  in  captivity, 
which  at  once  introduce  disturbances,  I  incline  to  the  idea  that 
egeria  has  not  got  the  right  periodicity  for  northern  climates. 
If  it  could  arrange  its  life  so  that  the  population  consisted  either 
of  young  larvae,  or  perhaps  of  thoroughly  formed  pupae11  at 
the  onset  of  winter,  it  might,  for  any  obvious  reason  to  the 
contrary,  be  able  to  live  in  England.  It  is  irregularly  "poly- 
voltine,"  as  the  silk-worm  breeders  say,  and  as  soon  as  a  little 
warmth  encourages  it,  a  new  generation  starts  into  being,  which 
if  the  frost  comes  at  an  untimely  moment,  is  immediately 

11  My  experience  agrees  with  that  of  Mr.  H.  Williams  (Ent.  Rec.,  VIII,  1896, 
p.  181)  that  pupae,  well-formed,  can  stand  considerable  frost;  but  I  used  to  find 
that  half -grown  larvae  usually  died  if  unprotected,  and  I  believe  that  larvae  which 
attempted  to  pupate  in  warm  autumn  weather  and  then  got  caught  by  frosts, 
always  died.  Small  larvae  which  can  creep  into  shelter  at  the  bottom  of  the  plants 
survived,  and  I  expect  that  in  the  north  the  winter  is  usually  passed  in  that  state 
(see  also  Merrifield,  F.,  Ent.  Rec.,  VIII,  1896,  p.  168,  and  Carpenter,  J.  H.,  ibid.). 


CLIMATIC  VARIETIES  173 

destroyed.  Many  species  are  continually  throwing  off  indi- 
viduals which  feed  up  fast12  and  emerge  at  once  if  the  temperature 
permits,  and  I  imagine  a  species  of  Satyrid  wholly  or  largely 
represented  by  such  individuals  could  scarcely  survive  in  a 
country  which  had  a  hard  winter.  For  such  a  climate  some 
definite  periodicity  in  the  appearance  of  the  broods  may  well  be 
indispensable.  But  assuming  that  egeria  is  cut  off  from  cold 
climates  for  such  a  reason,  there  is  nothing  yet  to  connect  these 
habits  with  the  fulvous  colour,  and  until  breeding  can  be  carried 
out  on  a  satisfactory  scale  there  is  no  more  to  be  said. 

From  time  to  time  records  appear  of  individual  specimens 
more  or  less  fulvous  being  caught  in  southern  England,  especially 
in  the  New  Forest.13  It  would  be  interesting  to  know  what 
offspring  such  individuals  might  produce.  From  the  evidence 
now  given  some  notion  both  of  the  strength  and  the  weakness 
of  the  case  considered  as  one  of  continuous  climatic  variation 
can  be  formed.  I  know  no  other  equally  satisfactory.  Whether 
or  not  definite  mixture  of  the  intermediates  with  either  of  the 
extremes  will  be  proved  to  occur,  the  case  differs  materially  from 
those  considered  in  the  last  chapter  in  the  fact  that  at  all  events 
there  is  no  general  overlapping  of  forms.  In  a  species  so  little 
given  to  wandering,  overlapping  could  indeed  scarcely  be  expected 
to  occur.  It  is  this  circumstance  which  makes  the  species 
preeminently  suitable  as  a  subject  for  the  study  of  climatic 
influences,  and  I  trust  that  entomologists  with  the  right  oppor- 
tunities may  be  disposed  to  explore  the  facts  further. 

Just  as  many  species,  like  egeria,  have  varieties  which  can 
be  regarded  as  adapted  to  northern  and  southern  regions,  so 
there  are  also  several  which  have  lowland  and  Alpine  forms  quite 
distinct  from  each  other.  Every  such  case  presents  an  example 
of  the  problem  we  have  been  considering.  As  the  collector 
passes  from  the  plains  to  the  Alpine  region,  how  will  he  find  the 

u  Some  most  unlikely  species  do  this.  I  once  had  a  larva  of  Parnassius  delizis, 
found  at  about  5,500  feet,  which  emerged  late  in  the  autumn  (in  October  I  believe), 
a  season  at  which  it  must  have  perished  in  its  own  country. 

13  See,  for  examples,  Barrett,  G.  C.,  Lepidoptera  of  the  Brit.  Islands,  I.  1893, 
p  229;  also  Grover,  W.,  Ent.  Rec.,  IX,  1897,  p.  314;  Williams,  H.,  Proc.  Ent.Soc., 
1898,  who  reared  several  specimens  from  the  New  Forest  which  would  pass  for 
Bretons,  though  the  rest  of  the  family  were  true  egerides. 


174  PROBLEMS  OF  GENETICS 

transition  from  one  form  to  the  other  effected?  Does  the  low- 
land form  give  place  to  the  Alpine  form  suddenly,  with  a  region 
in  which  the  two  are  mixed,  or  will  he  find  a  zone  inhabited  by 
an  intermediate  population?  I  have  spent  a  good  deal  of  time 
examining  the  facts  in  the  case  of  Pieris  napi  and  its  Alpine 
female  variety  bryoniae,  and  though  there  are  many  compli- 
cations which  still  have  to  be  cleared  up,  no  doubt  is  possible 
as  to  the  main  lines  of  the  answer.  If  in  any  valley  in  the  Alps 
inhabited  by  both  napi  and  bryoniae  the  collector  catches  every 
specimen  he  can,  beginning  at  the  bottom  and  working  up  to 
7,000  feet,  he  will  at  first  get  nothing  but  napi.  At  about  2,500 
feet,  he  may  catch  an  occasional  bryoniae  flying  with  the  napi. 
After  3,000  feet  napi  usually  ceases,  and  only  bryoniae  are  found. 
As  an  exception  a  colony  of  napi  may  be  met  with  at  much 
greater  heights.  I  once  found  them  in  numbers  at  about  6,000 
feet.14  Not  only  were  they  free  from  any  trace  of  modification 
in  the  direction  of  bryoniae,  but  they  were  of  the  thoroughly 
southern  type  of  napi,  being  a  late  brood  of  that  large  and  very 
pale  kind  (meridionalis)  almost  destitute  both  of  dark  veining 
above  and  of  green  veining  below,  which  are  common  on  the 
shores  of  Lago  Maggiore  and  in  other  hot  southern  localities. 
Not  far  off  at  the  same  level  were  typical  bryoniae  in  fair  abund- 
ance. Occasionally  an  intermediate  may  be  met  with.  I  have 
taken  a  few,  for  example,  at  Macugnaga  and  at  Fobello.  These, 
however,  in  my  experience  are  rarities  in  the  Alps.  Fleck15 
gives  notes  on  the  distribution  in  Roumania  which  shows  the 
same  state  of  things.  The  lowland  form  is  not  transformed 
though  found  at  great  heights,  and  at  Azuga  (nearly  3,000  feet) 
bryoniae  occurs  with  only  occasional  "flavescens,"  viz.,  inter- 
mediates of  the  second  brood. 

If  this  were  all  the  evidence  we  should  be  satisfied  that  the 
lowland  and  Alpine  types  keep  practically  distinct,  overlapping 
occasionally,  but  rarely  interbreeding.  The  problem  would 
remain,  how  is  the  distinctness  of  the  two  types  maintained  in 
the  region  of  overlapping?  Nowadays,  I  suppose,  we  should 

14  Above  the  Tosa  falls. 

16  Bui.  Soc.  Sciinte,  VIII,  1899,  p.  691. 


CLIMATIC  VARIETIES  175 

incline  to  answer  this  question  by  reference  to  segregation,  and 
perhaps  by  an  appeal  to  selective  mating.  The  suggestion  that 
segregation  does  take  place  is  certainly  true  to  some  extent. 
There  are,  however,  difficulties  in  the  way,  and  the  whole  subject 
is  one  of  great  complexity.  My  own  experiments  were  made  in 
pre-Mendelian  times  and  were  not  arranged  with  the  simplicity 
which  we  now  know  to  be  essential.  The  results  are  neither 
extensive  enough  nor  clear  enough  to  settle  the  many  collateral 
questions  which  have  to  be  considered,  and  the  work  ought  to 
be  done  again.  Nevertheless,  some  notes  of  the  observations 
may  have  a  suggestive  value. 

When  I  began,  I  did  not  sufficiently  appreciate  that  the 
"napi"  group,  omitting  the  North  American  forms,  and  the 
Asiatic  representatives,  has  at  least  three  chief  types  in  western 
Europe.  The  differences  we  have  to  deal  with  are  manifested 
by  the  females  only,  so  in  this  account  particulars  as  to  the  males 
are  omitted  for  the  most  part.  These  are  (i)  our  own  British 
napi;  (2)  the  form  found  in  the  south,  from  the  Loire  downwards, 
and  in  the  Italian  Alps,  which  I  think  may  be  spoken  of  as 
meridionalis;  (3)  bryoniae,  which  is  a  form  clearly  recognizable 
in  the  female  only,  and  is  found  only  in  the  arctic  regions  and 
in  the  Alps  above  2,500  feet.  The  first  two  have  several  broods, 
two,  three,  or  more,  according  to  opportunity,  and  the  first 
brood  is  different  from  the  later  ones.  In  napi  the  markings  on 
the  upper  surface  are  a  dark  grey  but  in  meridionalis  they  are  a 
pale  silvery  grey  and  much  less  extensive.  In  the  later  broods 
of  napi  there  is  much  less  general  irroration  of  the  veins,  and  the 
spots  stand  out  as  more  defined  and  blacker.  These  differences 
vary  greatly  in  degree  of  emphasis.  In  meridionalis  the  later 
broods  are  entirely  different  from  the  first.  Instead  of  having 
silvery  markings  they  have  the  ground  colour  quite  white,  with 
the  spots  large  and  a  full  black.  On  the  under  side  of  the  hind 
wings  the  usual  green  veins  are  almost  absent,  and  I  have  seen 
individuals  which  could  scarcely  be  distinguished  from  rapae. 
To  these  later  broods  the  term  napaeae  is  sometimes  applied, 
but  I  here  use  meridionalis  for  the  southern  race  in  general  as 
applicable  to  all  broods. 


176  PROBLEMS  OF  GENETICS 

The  female  bryoniae  is  totally  unlike  the  others.  The  ground 
colour  is  a  full  yellow,  and  each  nervure  is  thickly  irrorated  with 
a  brown  pigment  often  spreading  so  far  as  to  hide  the  ground 
almost  entirely  in  the  forewings.  The  males  corresponding  with 
these  females  are  not  certainly  distinguishable  from  those  of  our 
own  napi.  Both  sexes  have  the  green  veining  of  the  underside 
of  the  hind  wing  fully  developed,  rather  more  than  is  usual  in  the 
lowland  races,  but  this  is  not  really  diagnostic  of  the  variety. 
The  first  serious  difficulty  arises  in  regard  to  the  second  brood 
of  bryoniae.  It  is  stated  that  there  is  only  one  brood,16  but  I  feel 
fairly  sure  that  a  second  brood  is  sometimes  produced,  and  that 
the  females  with  a  yellow  ground  and  diminished  irroration  of  the 
veins,  not  very  uncommon  in  the  Italian  Alps  in  July  to  August, 
are  generally  representatives  of  it.  Such  insects  would  of  course 
be  classed  with  bryoniae  in  collections. 

My  experiments  began  with  eggs  of  true  bryoniae  females 
caught  at  about  2,500  feet  early  in  July.  These  emerged  in 
August— September  as  intermediates  with  yellow  ground  and 
about  half  as  much  black  on  the  upper  surface  as  bryoniae. 
They  are  exactly  like  the  intermediates  usually  found  in  nature 
and  in  the  light  of  later  experience  I  regard  them  as  natural  FI 
forms,  and  I  think  the  mothers  had  been  fertilised  by  napi  males, 
though  I  admit  that  in  view  of  the  rarity  of  natural  intermediates 
there  is  a  difficulty  in  this  suggestion.  Three  of  these  females 
were  mated  with  males  raised  from  thorough  meridionalis 
females,  and  three  families  were  produced.  Two  of  them 
showed  distinct  evidence  of  segregation,  some  being  yellow  and 
some  white  with  various  intergrades,  some  being  no  blacker  than 
meridionalis  and  some  ranging  up  to  a  dark  intermediate  type. 
Part  emerged  in  the  same  autumn;  and  part  overwintered,  emerg- 
ing as  the  spring  meridionalis  or  as  the  peculiar  type  which  I  after- 
wards learnt  to  know  as  the  spring  FI  form.  The  distinctions 
were  fairly  sharp  between  the  several  forms.  But  the  offspring 
of  the  third  female  gave  a  series  practically  continuous  from 

16  The  fact  that  Weismann  by  heating  pupae  obtained  only  one  autumn  speci- 
men seems  to  me  to  show  rather  that  a  second  brood  can  be  produced  than  that  it 
cannot,  which  is  the  inference  usually  drawn. 


CLIMATIC  VARIETIES  177 

meridionalis  to  the  FI  type.  The  work  of  subsequent  years 
gave  results  similarly  irregular  which  could  only  be  described 
adequately  at  great  length.  The  outcome  may  however  be 
summed  up  in  the  statement  that  there  is  evidence  that  both 
the  yellow  ground  and  the  dark  veining  are  due  to  factors,  but 
that  there  are  several  of  these  and  that  imperfect  segregation 
is  not  uncommon,  producing  various  reduction-stages.  The 
yellow  ground  may  be  due  to  one  factor,  and  the  several  shades 
may  be  the  result  of  irregularities  in  dominance ,  but  the  black 
markings  when  fully  developed  cannot  I  think  be  the  result  of 
less  than  three  factors,  one  for  the  basal  darkening,  one  for  general 
irroration,  and  one  for  the  margins.  Probably  also  the  enlarge- 
ment of  the  spots  is  produced  by  a  fourth  factor. 

There  was  not,  in  my  experience  any  great  difficulty  in  getting 
the  various  forms  to  pair  in  captivity.  Some  attempts  were 
made  to  see  whether  individuals  of  either  type  selected  mates 
of  their  own  type  in  preference  to  those  of  the  other,  but  the 
results  were  inconclusive.  There  were  some  indications  of  such 
a  preference;  though,  from  the  impossibility  of  judging  how  much 
of  this  may  be  due  to  other  circumstances,  I  could  not  come  to 
a  positive  conclusion  on  the  rather  meagre  evidence. 

Recently  Schima17  has  given  a  careful  and  detailed  account 
of  all  the  forms  found  in  Lower  Austria  which  he  enumerates 
under  14  distinct  varietal  names.  He  gives  full  references  to 
previous  accounts,  especially  to  the  beautiful  plates  lately  pub- 
lished by  Roger  Verity.18  Examination  of  these  and  of  my  own 
specimens  strongly  suggests  that  the  several  forms  are  due  to 
the  recombination  of  the  factors  I  have  named.  Among  those 
which  I  have  bred  are  representatives  of  most  if  not  all  the  types 
enumerated  by  Schima  in  addition  to  other  curious  forms.  For 
example  I  have  bryoniae  markings  on  a  ground  practically  white; 
the  dark  veins  with  spots  almost  obsolete;  meridionalis  on  a 
yellow  ground;  the  intermediate  amount  of  black  on  a  white 
ground,  etc.  The  last-named  may  occur  wild  and  I  have  one 
from  Macugnaga  as  well  as  one  given  me  by  Mr.  F.  Gayner  from 
Lulea  (Lapmark). 

17  Schima,  K.,  Verh.  Zool.  boi.  Ges.  Wien,  LX,  1910,  p.  268. 

18  Rhopalocera  Palaearctica,  Florence,  1905-11,  especially  PI.  XXXII. 
13 


178  PROBLEMS  OF  GENETICS 

To  obtain  really  exact  knowledge  of  the  number  of  factors 
and  their  properties  it  would  be  necessary  to  repeat  the  work. 
After  the  beginning,  I  made  a  mistake  in  using  British  napi 
instead  of  meridionalis  and  the  results  were  much  confused 
thereby.  The  contrast  between  meridionalis  and  the  various 
dark  forms  is  much  greater  and  classification  of  the  types  would 
have  been  therefore  easier.  The  British  form  is  presumably 
meridionalis  plus  the  factor  for  the  basal  pigmentation.  The 
problem  is  greatly  complicated  by  the  differentiation  of  the 
seasonal  forms.  The  first  point  to  be  determined  is  whether 
bryoniae  is  capable  of  producing  a  second  brood  when  it  is  thor- 
oughly pure-bred,  and  whether  such  a  second  brood  is,  as  I 
suspect,  normally  intermediate  in  character. 

In  the  Alps  generally  there  is  no  definitely  intermediate 
population;  nor  I  believe,  is  any  such  population  met  with  in 
the  north  where  the  arctic  bryoniae  meets  napi,  but  as  to  this 
I  have  no  precise  information.  One  curious  fact,  however,  must 
be  mentioned,  namely  that  there  is  a  population  that  can  prob- 
ably be  so  described  with  fairness  established  at  Modling  near 
Vienna.  This  is  not  in  any  sense  an  Alpine  locality,  and  does 
not,  as  I  am  told,  differ  in  any  obvious  way  from  the  other  sub- 
urbs of  Vienna.  Dr.  H.  Przibram  was  so  good  as  to  send  me  a 
set  taken  at  this  place,  representing  a  second  brood,  and  they 
were  decidedly  heterogeneous,  ranging  from  an  intermediate 
form  such  as  bryoniae  fertilised  by  napi  usually  produces,  to  a 
light  yellowish  second-brood  type  with  little  dark  pigment. 
There  are  also  two  actual  bryoniae.  Whether  true  napi  also 
occur  there  I  do  not  know,  but  I  have  no  doubt  they  do.  It 
would  be  well  worth  while  to  investigate  the  Modling  population 
statistically,  and  to  breed  from  the  intermediates  which  might 
not  impossibly  prove  to  be  heterozygotes.  There  are  also  records 
of  such  intermediates  being  occasionally  found  in  some  parts  of 
Ireland,  in  the  north  of  Scotland,  and  in  south  Wales,19  but  I  do 
not  know  of  any  regular  colony  of  these  forms.  We  can  scarcely 
avoid  the  inference  that  one  or  more  of  the  factors  which  make 
up  bryoniae  may  be  carried  by  these  intermediates.  It  is  not 

19  See  figures  in  Barrett,  G.  C.,  Lepidoptera  of  Brit.  Islands,  I,  pt.  3,  p.  25. 


CLIMATIC  VARIETIES  179 

clear  why  their  interbreeding  does  not  produce  actual  bryoniae 
occasionally.  If  this  occurred,  the  probability  is  that  the  fact 
would  be  known  to  collectors,  at  least  in  the  British  localities. 
The  absence  of  true  bryoniae  must,  I  think,  be  taken  to  mean 
that  some  essential  factor  is  absent  from  these  intermediates. 

To  sum  up  the  evidence,  the  facts  that  are  clear  may  be  thus 
enumerated : 

1.  Napi  and  bryoniae,  or  in  the  Italian  Alps,  napaeae  and 
bryoniae  frequently  meet  each  other. 

2.  They  cross  without  difficulty,  producing  fertile  offspring. 

3.  But  in  the  levels  at  which  they  overlap  there  is  no  inter- 
mediate population,  and  only  occasional  intermediate  individuals. 

4.  In  certain  parts  of  the  distribution  of  napi  similar  inter- 
mediates sometimes  occur,  and  at  one  place  (Modling)  they  are 
so  frequent  as  apparently  to  constitute  a  colony. 

5.  As  to  the  genetic  relations  of  the  two  forms  there  is  no 
complete  certainty.     Indications  of  segregation  have  been  ob- 
served in  some  cases,  but  there  are  several  factors  concerned  and 
they  are  liable  to  some  disintegration. 

Another  form  in  which  I  tried  to  investigate  the  same  problem 
is  Coenonympha  arcania,  which  has  one  Alpine  form  known  as 
darwiniana,  and  another,  satyrion.  In  calling  satyrion  a  form  of 
arcania  I  follow  Staudinger  and  other  authorities,  but  I  have 
never  been  quite  satisfied  that  it  should  be  so  regarded.  The 
differences  between  arcania.  and  darwiniana  are  essentially  dif- 
ferences of  degree;  C.  arcania  occurs  in  places  where  there  is 
cover,  and  reaches  up  the  valleys  usually  as  high  as  the  mixed 
woods  of  deciduous  trees,  which  is  about  2,500  feet.  The  variety 
darwiniana,  on  the  contrary,  is  an  insect  of  treeless  hillsides,  and 
I  regard  it  as  a  dwarf  and  possibly  a  stunted  form.  It  would  not 
greatly  surprise  me  to  find  that  with  the  application  of  good 
conditions  arcania  could  be  raised  from  darwiniana  eggs,  or  that 
if  arcania  larvae  were  starved  they  might  give  rise  to  darwiniana 
butterflies.  I  have  been  unsuccessful  in  trying  to  rear  the  species, 
having  lost  the  larvae  by  disease.  Usually  one  does  not  catch 
arcania  and  darwiniana  on  the  same  ground,  and  as  Festuca  ovina 
— a  typically  hill-side  grass — is  a  common  food-plant  of  dar- 


i8o  PROBLEMS  OF  GENETICS 

winiana  there  can  be  little  doubt  that  arcania  feeds  on  some  other 
grass,  probably  woodland  species.  Colonies  of  arcania  of  varying 
size  and  brightness  are  commonly  found,  and  though  a  sample  of 
arcania,  finely  grown,  from  a  warm  Italian  wood,  presents  a 
striking  contrast  with  darwiniana  from  an  Alpine  pasture,  one 
certainly  may  get  samples  which  fill  all  the  gradations.  Gen- 
erally the  sample  from  a  given  locality  is  fairly  homogeneous. 

Of  satyrion  I  have  little  personal  experience.  I  only  twice 
found  it,  namely  at  Zinal,  and  at  Hallstatt  in  Austria,  but  it 
occurs  at  Zermatt,  Arolla,  and  in  several  Swiss  localities  above 
5,000  feet,  and  I  understand  that  it  is  the  typical  Alpine  form  in 
the  Engadine.  With  its  darkened  colour  and  reduced  size  it 
might  well  be  expected  to  be  a  still  further  stunted  form  of 
darwiniana.  Yet  I  have  never  found  the  one  succeed  to  the 
other  at  the  higher  levels.  If  darwiniana  appears  when  Alpine 
conditions  are  reached  in  a  valley  it  will  be  met  with  up  to  the 
highest  level  at  which  such  butterflies  live.  Tutt  was  of  opinion 
that  satyrion  is  a  distinct  species.20  I  once,  at  the  top  of  the  Vor- 
derrheinthal  caught  a  sample  of  darwiniana  a  few  of  which  (males) 
were  so  dark  and  had  the  eye  spots  so  poorly  developed  that  they 
looked  like  transitions  to  satyrion.  Otherwise  I  never  found 
any  such  transitional  forms  and  they  are  certainly  exceptional. 
There  is  further  a  record21  of  satyrion  having  been  taken  flying 
with  arcania.  This  was  near  Susa,  at  about  2,000  feet  I  infer. 
Mr.  H.  E.  Page  has  similar  specimens  from  Caud  and  from  St. 
Anton  (Arlberg).  The  females,  however,  both  of  mine  and  of 
Mr.  Page's  samples  are  a  pale  brown,  quite  unlike  the  females 
both  of  arcania  and  of  the  dark  Zinal  satyrion.  The  difficulty 
thus  raised  has  not  I  think  yet  been  considered  by  the  authorities, 
and  it  is  possible  that  the  Alpine  forms  of  arcania  are  in  reality 
three,  not  two. 

The  evidence  taken  together  suggests,  I  think,  that  darwin- 
iana is  related  to  arcania  much  as  so  many  of  the  Alpine  varieties 

20  Tutt,  J.  W.,  Ent.  Rec.,  XVIII,  1905,  p.  5.     In  the  same  place  he  states  that 
on  the  Mendel  Pass  arcania  "runs  into"  darwiniana  and  that  in  the  Tyrolean 
localities  the  transition  is  especially  evident.     Wheeler  (ibid.,  XIII,  1901,  p.  121) 
expresses  the  contrary  opinion,  that  satyrion  does  grade  to  arcania. 

21  H.  Rowland-Brown,  Ent.  Rec.,  XI,  1899,  p.  293. 


CLIMATIC  VARIETIES  181 

of  plants  are  to  the  well-developed  individuals  of  the  lower 
levels.  I  do  not  anticipate  that  factorial  differences  will  be 
found  in  these  insects,  and  it  is  by  no  means  impossible  that  the 
distinctions  between  them  are  the  direct  consequences  of  altered 
conditions.  The  relations  of  arcania  to  satyrion  are  more  doubt- 
ful, and  in  that  case  a  factorial  difference  may  at  least  be  sus- 
pected. 

The  species  of  the  genus  Setina  have  Alpine  forms  which 
agree  in  possessing  a  characteristic  extension  of  the  black  pigment 
to  form  radiating  junctions  between  the  spots  on  the  wings. 
Speyer,  who  discussed  the  interrelations  of  these  forms  in  detail,22 
lays  stress  on  the  absence  of  genuine  transitional  forms  between 
aurita  and  the  variety  ramosa.  Both  are  mountain  insects  but 
ramosa  extends  to  levels  higher  than  that  at  which  aurita  ceases, 
which  is  about  4,000  feet.  The  two  forms  are  often  found  flying 
together.  Speyer  says  that  his  brother  searched  diligently  for 
transitional  forms  at  the  level  of  overlapping,  but  found  none, 
so  that  at  least  they  may  be  regarded  as  rare.  The  variety 
ramosa  is  not  infrequent  at  much  lower  levels  (e.  g.,  Chiavenna, 
1,020  feet;  Reussthal,  1,500  feet)  and  extends  as  high  as  the 
permanent  snows.  In  the  British  Museum  collection,  however, 
I  have  seen  several  that  I  should  regard  as  transitional.  Speyer 
perhaps  would  have  classed  as  ramosa  all  in  which  the  spots  of 
the  central  field  were  united,  and  it  is  by  no  means  unlikely  that 
breeding  would  prove  such  individuals  to  be  heterozygous.23 

22  Speyer,  Stettiner,  Ent.  Ztg.,  XXXI,  1870,  p.  63. 

23  In  regard  to  the  closely  analogous  case  of  Spilosoma  lubricipeda,  Standfuss 
makes  a  similar  statement.     He  bred  the  type  on  a  large  scale  with  the  radiate  form 
which  he  calls  intermedia,  and  says  that  in  four  years  of  miscellaneous  crossing  he 
never  obtained  really  transitional  forms.     Nevertheless  after  examining  large  series, 
especially  those  of  Mr.  W.  H.  B.  Fletcher,  I  came  to  the  conclusion  that  several 
might  be  so  classed,  but  I  am  quite  prepared  to  find  that  such  specimens  are  hetero- 
zygous.    (See  Standfuss,  Handb.  d.  Gross- Schmet.,  1896,  p.  307.)     It  is  by  no  means 
unlikely  that  various  dark  forms  of  lubricipeda  correspond  with  a  progressive  series 
of  factorial  additions.     Many  of  the  stages  have  been  named,  and  of  these  the  most 
definite  are  the  intermedia  of  Standfuss  (probably  =  eboraci  of  Tugwell)  and  the 
very  dark  Zatima  of  Heligoland,  in  which  only  the  thorax,  the  nervures  and  a  small 
field  in  the  forewings  remain  yellow.     A  form  was  bred  by  Deschange  from  Zatima 
in  which  even  the  field  in  the  forewing  is  obliterated.     The  exact  circumstances  in 
which  Zatima  occurs  in  Heligoland  would  be  worthy  of  special  investigation,  for  the 


1 82  PROBLEMS  OF  GENETICS 

There  can  scarcely  be  a  doubt  that  the  distinction  between 
aurita  and  ramosa  is  factorial,  the  radiate  ramosa  probably  having 
the  factor  for  striping.  In  support  of  this  view  may  be  men- 
tioned the  observation  of  Boisduval,24  respecting  a  gynandro- 
morphous  individual,  which  was  aurita  male  on  one  side,  and 
ramosa  female  on  the  other.  Speyer  makes  another  excellent 
comment.  He  points  out  that  the  simple  notion  that  the  radi- 
ation is  a  mere  extension  of  pigmentation  consequent  on  the 
climate  of  the  higher  levels,  will  not  fit  the  facts  very  easily, 
because  the  size  of  the  spots  varies  greatly  in  aurita  itself  at  any 
level,  and  lowland  specimens  may  actually  have  more  black 
confined  to  the  spots  alone  than  some  ramosa  possess  on  spots  and 
lines  combined.25 

The  two  Salamanders,  S.  maculosa  and  its  Alpine  form  atra, 
might  not  improbably  furnish  evidence  bearing  on  the  same 
problem.  The  two  are  of  course  very  distinct,  not  merely  in 
colour  (maculosa  being  spotted  with  yellow  or  orange  while  atra 
is  entirely  black)  but  also  in  the  mode  of  reproduction,  a  feature 
to  which  reference  will  be  made  in  the  next  chapter.  I  cannot, 
however,  find  any  evidence  as  to  the  overlapping  of  the  two  forms. 
S.  atra  occurs  from  about  3,000  feet  or  somewhat  less,  and  reaches 
great  elevations  in  the  Eastern  Alps,  but  I  do  not  know  if  the 
two  forms  ever  occur  in  the  same  localities.  Leydig,26  Boulenger,27 
and  most  modern  authorities  regard  the  two  types  as  distinct 
species,  but  they  are  in  any  case  closely  allied,  and  it  would  be  of 
interest  to  have  exact  knowledge  of  their  geographical  delimi- 
tations. 

The  reader  who  has  considered  the  cases  adduced  will  ap- 
preciate the  difficulties  which  must  be  faced  in  any  attempt  to 

normal  lubricipeda  in  also  found  on  the  island.  For  references  as  to  the  British 
occurrences  see  especially,  Hewett,  W.,  Naturalist,  1894,  p.  353.  As  to  Zatima  see 
especially  Krancher,  Soc.  Ent.,  II,  1887-8,  p.  26.  I  am  indebted  to  Dr.  Hartlaub 
for  information  as  to  the  Heligoland  types. 

24  Boisduval,  Bull.  Soc.  Ent.  Fr.,  Ill,  1834,  p.  5. 

25  The  systematics  of  Setina  have  been  much  controverted,  but  no  one  I  believe 
doubts  that  aurita  and  ramosa  are  forms  of  one  species.     See  also  Chapman,  A.  T., 
Ent.  Rec.,  XIII,  1901,  p.  139. 

™Arch.  Naturg.,  33,  1867,  p.  116. 

27  Brit.  Mus.  Cat.,  Batrachia  Gradientia,  1882. 


CLIMATIC  VARIETIES  183 

account  for  the  facts  in  a  rational  way.  As  always  in  a  problem 
of  Evolution,  two  separate  questions  have  to  be  answered. 
First  how  did  the  form  under  consideration  come  into  existence, 
and  secondly,  how  did  it  succeed  in  maintaining  itself  so  as  to 
become  a  race?  The  evidence  from  the  local  forms,  though  very 
far  from  giving  complete  answers  to  either  of  these  questions 
definitely  refutes  the  popular  notion  that  a  new  race  comes  into 
existence  by  transformation  of  an  older  race.  If  a  gradual  mass- 
transformation  of  this  kind  took  place  we  should  certainly  expect 
that  when  two  types,  nearly  allied  and  capable  of  interbreeding, 
overlap  each  other  in  their  geographical  distribution,  a  normally 
intermediate  population  would  exist.  If  each  type  can  main- 
tain itself,  and  if  each  came  into  existence  by  gradual  transfor- 
mation, then  there  must  have  been  an  intermediate  capable  of 
existing  and  maintaining  itself  as  a  population;  and  if  this  had 
ever  been,  surely  in  the  region  of  overlapping,  that  intermediate 
population  should  continue.  Especially  should  such  a  population 
be  found  when  the  two  extreme  types  are  adaptational  forms  and 
the  region  of  overlap  is  a  region  of  intermediate  conditions. 
But  of  the  examples  we  have  examined  there  is  only  one,  that  of 
Pararge  egeria  and  egerides,  which  can  at  all  be  so  interpreted, 
and  even  in  that  case  it  is  not  impossible  that  more  minute  ob- 
servation would  reveal  discontinuity  between  the  extremes 
and  the  admittedly  normal  intermediate  population.  Granting 
provisionally  however  that  this  example,  as  it  stands,  is  con- 
sistent with  the  conventional  theory  of  evolution,  I  know  not 
where  we  should  look  for  another  case  equally  good.  When  the 
distinctions  are  produced  by  direct  influence  of  conditions  oper- 
ating during  the  lifetime  of  the  individuals,  examples  of  inter- 
mediate populations  occupying  the  areas  of  intermediate  con- 
ditions can  no  doubt  be  produced.  Many  turf -like  Alpine 
plants,  for  instance,  if  protected  from  exposure  and  properly 
nourished  can  grow  as  large  as  those  of  the  same  species  found  in 
the  valleys,  and  in  the  case  of  such  quantitative  effects,  inter- 
mediate conditions  can  doubtless  produce  intermediate  characters. 
Even  these  examples  however  are  not  very  abundant,  and 
often  the  intermediate  locality  has  not  a  form  intermediate 


184  PROBLEMS  OF  GENETICS 

between  those  of  the  two  extreme  localities,  but  some  third 
form  distinct  from  either.  This  is  the  case  for  instance  in  the 
fauna  of  brackish  waters.  We  are  taught  to  believe  that  the 
fresh  water  fauna  was  evolved  from  the  marine  fauna,  which 
it  well  may  have  been;  but  as  students  of  Crustacea  and  Mollusca 
know  familiarly,  the  brackish  water  forms  are  not  as  a  rule  inter- 
mediates between  fresh  water  species  and  sea  species,  but  more 
usually  they  are  special  forms  belonging  to  the  brackish  waters, 
with  the  peculiar  property  that  they  can  tolerate  a  great  range  of 
conditions,  and  live  without  ostensible  variation  in  waters  of 
most  various  compositions  and  densities,  which  very  few  marine 
or  fresh  water  species  are  able  to  do. 

Sometimes  the  distinction  between  local  races,  as  in  Rham- 
phocoelus  passerinii  and  icteronotus  may  be  regarded  with  con- 
fidence as  due  to  one  simple  Mendelian  factor  possessed  by  one 
race  and  absent  from  the  other,  but  I  think,  more  often,  as  in 
Colaptes  or  in  the  varieties  of  Pieris  napi,  the  existence  of  several 
distinct  factors  is  to  be  inferred.  As  we  have  seen,  the  races 
of  Colaptes  show  almost  beyond  doubt  that  in  different  areas  at 
least  three  distinct  factorial  combinations  can  be  perpetuated 
as  races. 

In  the  distribution  of  variability  we  find,  I  think,  some  hint 
as  to  the  steps  by  which  the  phenomena  under  consideration 
have  come  to  their  present  stage,  and  I  am  disposed  to  regard 
the  facts  so  well  attested  in  the  case  of  our  own  melanic  moths 
as  a  true  indication  of  the  process.  Following  this  indication 
we  should  regard  the  change  in  the  character  of  a  population 
as  beginning  sporadically,  by  the  appearance  of  varying  indi- 
viduals, possibly  only  one  varying  individual,  in,  it  may  be,  one 
place  only.  As  to  why  a  variety  should  increase  in  numbers  we 
have  nothing  but  mere  speculation  to  offer,  and  for  the  present 
we  must  simply  recognise  the  fact  that  it  may.  That  such  sur- 
vival and  replacement  may  reasonably  be  taken  as  an  indication 
that  the  replacing  race  has  some  superior  power  of  holding  its 
own  I  am  quite  disposed  to  admit.  Nevertheless  it  seems  in 
the  highest  degree  unlikely  that  the  outward  and  perceptible 
character  or  characters  which  we  recognise  as  differentiating  the 


CLIMATIC  VARIETIES  185 

race  should  be  the  actual  features  which  contribute  effectively 
to  that  result. 

In  discussions  of  geographical  distribution  in  relation  to 
problems  of  origin  it  is  generally  said  that  very  nearly  allied 
species  usually  occupy  distinct  areas,  while  other  competent 
observers  state  the  exact  contrary.  Lately,  for  example,  Dr. 
R.  G.  Leavitt28  has  published  an  important  collection  of  evidence 
upholding  the  latter  proposition,  taken  chiefly  from  the  botanical 
side,  showing  how  in  numerous  genera  two  or  more  closely  allied 
species  coexist,  frequently  without  intermediates,  in  the  same 
localities,  and  may  even  be  thus  found  in  company  throughout 
their  distribution.  The  difference  of  opinion  evidently  arises 
from  a  confusion  as  to  the  sense  in  which  the  term  "species'* 
is  understood  and  applied.  Leavitt,  for  example,  is  avowedly 
following  Jordan  and,  among  moderns,  Sargent,  in  applying 
a  close  analysis,  and  denoting  as  species  all  forms  which  are 
distinct  and  breed  true.  Against  this  use  of  the  term  I  know 
no  valid  objection29  but  it  must  be  obvious  that  if  others  follow 
a  different  practice  confusion  may  result  when  observations  are 
summarised  in  general  statements.  We  will  consider  this  subject 
again  in  another  place,  but  here  it  may  be  sufficient  to  say  that 
there  can  scarcely  now  be  a  doubt  that  numbers  of  these  associ- 
ated species,  such  as  Jordan  discriminated,  represent  various 
combinations  of  the  presence  and  absence  of  Mendelian  factors. 
This  does  not  in  any  way  weaken  the  argument  which  Leavitt 
founds  upon  the  facts,  namely,  that  the  observed  distribution 
of  these  forms  is  consistent  with  the  supposition  of  an  evolution 
largely  discontinuous. 

On  the  other  hand,  those  who  have  come  to  the  opinion  that 
nearly  allied  species  generally  occupy  distinct  ground  are  pre- 
sumably more  impressed  by  the  characters  differentiating  the 
geographically  distinct  or  adaptational  races,  seeing  that  genuine 
intermediates  between  them  are  less  commonly  found.  Those 
geographical  races  may  no  doubt  contain  various  differentiated 
forms;  but  when  all  live  together,  occasional  intermediates  are 

28  The  Geographical  Distribution  of  nearly  related  Species.     Amer.  Nat.,  XL  I . 
1907,  p.  207. 

29  See  later,  p.  242. 


186  PROBLEMS  OF  GENETICS 

usually  to  be  found  even  in  the  case  of  characters  habitually 
segregating.  These  segregating  forms  Jordan  would  certainly 
have  determined  as  species,  and  it  must  be  conceded  that  no 
physiological  definition  has  yet  been  drawn  which  consistently 
excludes  them. 


CHAPTER   IX 

THE  EFFECTS  OF   CHANGED   CONDITIONS:   ADAPTATION 

In  the  attempt  to  conceive  a  process  by  which  Evolution 
may  have  come  about,  the  first  phenomenon  to  be  recognized 
and  accounted  for  is  specific  difference.  With  that  recognition 
the  outline  of  the  problem  is  defined.  The  second  prerogative 
fact  is  adaptation.  Forms  of  life  are  on  the  whole  divided  into 
species,  and  these  species  on  the  whole  are  adapted  and  fit  the 
places  in  which  they  live.  To  many  students  of  Evolution, 
adaptation  has  proved  so  much  more  interesting  and  impressive 
than  specific  diversity  that  they  have  preferred  it  to  the  first 
place  in  their  considerations. 

Whether  this  is,  as  I  believe,  an  inversion  of  the  logical  order 
or  not,  there  is  one  most  serious  practical  objection  to  such 
preference,  that  whereas  specific  diversity  is  a  subject  which 
can  be  investigated  both  by  the  study  of  variation  and  by  the 
analytical  apparatus  which  modern  genetic  science  has  developed, 
we  have  no  very  effectual  means  of  directly  attacking  the  problems 
of  Adaptation. 

The  absence  of  any  definite  progress  in  genetics  in  the  last 
century  was  in  great  measure  due  to  the  exclusive  prominence 
given  to  the  problem  of  Adaptation.  Almost  all  debates  on 
heredity  centered  in  that  part  of  the  subject.  No  one  disputes 
that  the  adaptation  of  organisms  to  their  surroundings  is  one 
of  the  great  problems  of  nature,  but  it  is  not  the  primary  problem 
of  descent.  Moreover,  until  the  normal  and  undisturbed  course 
of  descent  under  uniform  conditions  is  ascertained  with  some 
exactness,  it  is  useless  to  attempt  a  survey  of  the  consequences 
of  external  interference;  nor  as  a  rule  can  it  be  even  possible  to 
decide  with  much  confidence  whether  such  interferences  have  or 
have  not  definite  consequences.  Those,  for  example,  who  de- 
bated with  enthusiasm  whether  acquired  characters  are  or  are 
not  transmitted  were  constantly  engaged  in  discussing  occur- 

187 


1 88  PROBLEMS  OF  GENETICS 

rences  which  we  now  know  to  be  ordinary  features  of  descent 
under  uniform  conditions,  and  the  origin  of  variations  which 
were  certainly  not  caused  directly  by  circumstances  at  all.  In 
the  absence  of  any  factorial  analysis,  or  of  any  conception  of  what 
factorial  composition  means  and  implies,  no  one  knew  what 
varieties  might  be  expected  from  given  parents.  The  appearance 
of  any  recessive  variety  was  claimed  as  a  consequence  of  some 
treatment  which  might  have  been  applied  to  the  parents.  There 
was  no  possible  standard  of  evidence  or  means  of  controlling  it, 
and  thus  the  discussion  was  singularly  unfruitful.  Before  we  can 
tell  how  the  course  of  descent  has  departed  from  the  normal,  we 
must  know  what  the  normal  would  have  been  if  we  had  let  alone. 
We  are  still  far  from  having  such  knowledge  in  adequate  measure, 
but  it  does  now  exist  in  some  degree,  and  we  are  steadily  approach- 
ing a  position  from  which  we  shall  be  able  to  form  fairly  sound 
estimates  of  the  true  significance  of  evidence  for  or  against  the 
proposition  that  environmental  treatment  can  produce  positive 
disturbances  in  the  physiological  course  of  descent. 

Thus  described,  the  field  for  consideration  is  very  wide. 
Though  the  effects  of  changed  conditions  were  especially  studied 
in  the  hope  of  solving  the  problem  of  adaptation  by  direct  ob- 
servation, that,  as  all  are  now  agreed,  is  but  a  part  of  a  more 
general  question.  We  must  ask  not  only  do  changed  conditions 
produce  an  adaptative  response  on  the  part  of  the  offspring,  but 
whether  they  produce  any  response  on  the  part  of  the  offspring 
at  all.  It  is  not  in  doubt  that  by  violent  means,  such  as  starvation 
or  poisoning  of  the  reproductive  cells,  effects  of  a  kind,  stunting 
and  deformity  for  instance,  can  be  made  evident,  just  as  similar 
effects  may  follow  similar  treatment  during  embryonic  or  larval 
life.  Apart  from  interferences  of  this  class,  are  there  any  that 
may  be  reasonably  invoked  as  modifying  the  course  of  inher- 
itance? 

No  epitome  of  the  older  evidence  for  the  inheritance  of 
adaptative  changes  is  here  required.  That  has  often  been  col- 
lected, especially  by  Weismann,  who  exposed  its  weaknesses  so 
thoroughly  as  to  carry  conviction  to  most  minds,  and  showed 
that  whether  the  phenomenon  occurs  or  not,  no  one  can  yet  prove 


ADAPTATION  189 

that  it  does.  Belief  in  these  transmissions,  after  being  almost 
universally  held,  was  with  singular  unanimity  abandoned.  This 
change  in  opinion,  though  doing  credit  to  the  faith  of  the  scientific 
community  in  evidential  reasoning,  is  the  more  remarkable 
inasmuch  as  the  strength  of  the  idea  was  not  derived  from  the 
minute  amounts  of  supposed  facts  now  demolished.  On  the 
contrary,  it  was  really  an  instinctive  deduction  from  a  wide 
superficial  acquaintance  with  the  properties  of  animals  and 
plants.  They  can  accommodate  themselves  to  circumstances. 
They  do  make  responses  sometimes  marvellously  appropriate 
to  demands  for  which  they  can  scarcely  have  been  prepared. 
What  more  natural  than  to  suppose  that  the  permanent  adapta- 
tions have  been  achieved  by  inherited  summation  of  such  re- 
sponses? No  one  had  actually  been  driven  to  believe  in  the 
inheritance  of  adaptative  changes  because  bitches  which  had 
been  docked  had  been  known  to  give  birth  to  tailless  puppies, 
or  because  certain  wheat  in  Norway  was  alleged  to  have  become 
acclimatized  in  a  few  generations.  Evidence  of  this  kind  was 
collected  and  produced  rather  as  an  ornamental  appendix  to  a 
proposition  already  accepted,  and  held  to  be  plainly  demon- 
strated by  the  facts  of  nature.  Looked  at  indeed  in  that  pre- 
liminary and  uncritical  way,  the  case  is  simply  overwhelming. 
Those  who  desire  to  see  how  strong  it  is  should  turn  to  Samuel 
Butler's  Life  and  Habit,  and  even  if  in  reading  they  reiterate  to 
themselves  that  no  experimental  evidence  exists  in  support  of 
the  propositions  advanced,  the  misgiving  that  none  the  less  they 
may  be  true  is  likely  to  remain.  Making  every  deduction  for 
the  fact  that  the  wonders  of  adaptation  have  been  grossly  ex- 
aggerated, and  that  marvels  of  fitness  and  correspondence  be- 
tween means  and  ends  have  grown  out  of  mere  anthropomorphic 
speculations,  there  is  much  more  left  to  be  accounted  for  than 
can  at  all  comfortably  be  accepted  as  the  product  of  happy 
accidents.  So  oppressive  are  these  difficulties  that  we  can  scarcely 
blame  those  who  imagine  that  the  study  of  heredity  is  primarily 
directed  to  the  problem  of  the  transmission  of  acquired  characters, 
a  preconception  still  almost  universal  among  the  laity. 

But  since  the  belief  in  transmission  of  acquired  adaptations 


190  PROBLEMS  OF  GENETICS 

arose  from  preconception  rather  than  from  evidence,  it  is  worth 
observing  that,  rightly  considered,  the  probability  should  surely 
be  the  other  way.  For  the  adaptations  relate  to  every  variety 
of  exigency.  To  supply  themselves  with  food,  to  find  it,  to  seize 
and  digest  it,  to  protect  themselves  from  predatory  enemies 
whether  by  offence  or  defence,  to  counter-balance  the  changes 
of  temperature,  or  pressure,  to  provide  for  mechanical  strains, 
to  obtain  immunity  from  poison  and  from  invading  organisms, 
to  bring  the  sexual  elements  into  contact,  to  ensure  the  dis- 
tribution of  the  type;  all  these  and  many  more  are  accomplished 
by  organisms  in  a  thousand  most  diverse  and  alternative  methods. 
Those  are  the 'things  that  are  hard  to  imagine  as  produced  by 
any  concatenation  of  natural  events;  but  the  suggestions  that 
organisms  had  had  from  the  beginning  innate  in  them  a  power 
of  modifying  themselves,  their  organs  and  their  instincts  so  as 
to  meet  these  multifarious  requirements  does  not  materially 
differ  from  the  more  overt  appeals  to  supernatural  intervention. 

The  conception,  originally  introduced  by  Hering  and  inde- 
pendently by  S.  Butler,  that  adaptation  is  a  consequence  or 
product  of  accumulated  memory  was  of  late  revived  by  Semon 
and  has  been  received  with  some  approval,  especially  by  F. 
Darwin.  I  see  nothing  fantastic  in  the  notion  that  memory 
may  be  unconsciously  preserved  with  the  same  continuity  that 
the  protoplasmic  basis  of  life  possesses.  That  idea,  though 
purely  speculative  and,  as  yet,  incapable  of  proof  or  disproof 
contains  nothing  which  our  experience  of  matter  or  of  life  at  all 
refutes.  On  the  contrary,  we  probably  do  well  to  retain  the 
suggestion  as  a  clue  that  may  some  day  be  of  service.  But  if 
adaptation  is  to  be  the  product  of  these  accumulated  experiences, 
they  must  in  some  way  be  translated  into  terms  of  physiological  and 
structural  change,  a  process  frankly  inconceivable. 

To  attempt  any  representation  of  heredity  as  a  product  of 
memory  is,  moreover,  to  substitute  the  more  obscure  for  the 
less.  Both  are  now  inscrutable ;  but  while  we  may  not  unreason- 
ably aspire  to  analyse  heredity  into  simpler  components  by  or- 
dinary methods  of  research,  the  case  of  memory  is  altogether 
different.  Memory  is  a  mystery  as  deep  as  any  that  even  psy- 


ADAPTATION  191 

chology  can  propound.  Philosophers  might  perhaps  encourage 
themselves  to  attack  the  problem  of  the  nature  of  memory  by 
reflecting  that  after  all  the  process  may  in  some  of  its  aspects 
be  comparable  with  that  of  inheritance,  but  the  student  of  gen- 
etics, as  long  as  he  can  keep  in  close  touch  with  a  profitable  basis 
of  material  fact,  will  scarcely  be  tempted  to  look  for  inspiration 
in  psychical  analogies. 

For  a  summary  of  the  recent  evidence  I  may  refer  the  reader 
to  Semon's  paper1  where  he  will  find  a  collection  of  these  obser- 
vations described  from  the  standpoint  of  a  convinced  believer. 
At  the  outset  one  cannot  help  being  struck  by  the  fact  that  of 
the  instances  alleged,  very  few,  even  if  authentic,  show  the  trans- 
mission of  acquired  modifications  which  can  in  any  sense  be  re- 
garded as  adaptative,  and  many  are  examples  not  so  much  of  a 
transmission  of  characters  produced  in  the  parents  as  of  variation 
induced  in  the  offspring  as  a  consequence  of  treatment  to  which 
the  parents  were  submitted,  the  parents  themselves  remaining 
apparently  unmodified.  No  one  questions  the  great  importance 
of  evidence  of  this  latter  class  as  touching  the  problem  of  the 
causes  of  variation,  but  it  is  not  obvious  why  it  is  introduced  in 
support  of  the  thesis  that  acquired  characters  are  inherited. 

It  is  most  difficult  to  form  a  clear  judgment  of  the  value  of 
the  evidence  as  a  whole.  To  doubt  the  validity  of  testimony 
put  forward  by  reputable  authors  is  to  incur  a  charge  of  obstinacy 
or  caprice;  nevertheless  in  matters  of  this  kind,  where  the  alleged 
phenomena  are,  if  genuine,  of  such  exceptional  significance,  belief 
should  only  be  extended  to  evidence  after  every  possible  source 
of  doubt  has  been  excluded.  We  believe  such  things  when  we 
must,  but  not  before.  At  the  very  least  we  are  entitled  to  require 
that  confirmatory  evidence  should  be  forthcoming  from  inde- 
pendent witnesses.  So  far  as  I  have  seen,  this  requirement  is 
satisfied  in  scarcely  any  of  the  examples  that  have  been  lately 
published,  and  until  it  is,  judgment  may  reasonably  be  suspended. 

In  some  cases,  however,  the  facts  are  not  doubtful.  Stand- 
fuss,  by  subjecting  pupae  of  Vanessa  urticae  to  cold,  produced 

1  Semon,  R.,  Der  Stand  der  Frage  nachder  Vererbung  erworbener  Eigenschaften, 
published  in  Fortschr.  der  naturw.  Forschung.,  Bd.  n,  1910. 


I92  PROBLEMS  OF  GENETICS 

the  now  well-known  temperature-aberrations  in  which  the  dark 
pigment  is  greatly  extended.  He  put  together  in  a  breeding-cage 
32  males  and  10  females  showing  this  modification  in  various 
degrees.  Two  of  these  females  died  without  leaving  young. 
Seven  produced  exclusively  normal  offspring.  From  the  eighth 
female  43  butterflies  were  bred,  and  of  these  there  were  four  (all 
males)  which  to  a  greater  or  less  extent  exhibited  the  aberrational 
form.2  The  mother  of  this  family  was  the  most  abnormal  of  the 
10  females  originally  put  in. 

Fischer's  experiment  with  Aretia  caja  was  on  similar  lines. 
From  pupae  which  had  been  frozen  almost  all  the  moths  which 
emerged  showed  aberrational  markings.  A  pair  of  these  mated 
and  produced  173  young  which  pupated.  Those  which  emerged 
early  were  all  normal,  but  of  those  which  emerged  late,  17  had 
in  various  degrees  abnormal  markings  like  those  of  the  parents.3 
In  neither  of  these  examples  is  there  any  question  as  to  the  facts. 
Both  observers  have  great  experience  and  give  full  details  of  their 
work. 

As  regards  Vanessa  urticae,  however,  it  must  be  recalled  that 
Fischer  himself  showed  that  in  Nymphalids  somewhat  similar 
aberrations  could  be  produced  both  by  heat  and  by  cold,  and 
even  by  centrifuging  the  pupae.  Frl.  von  Linden  produced  a 
transitional  form  of  the  same  aberration  in  V.  urticae  by  the 
action  of  carbonic  acid  gas.4  It  is  highly  probable  that  the  ap- 
pearance is  due  to  a  morbid  change,  perhaps  an  arrest  of  develop- 
ment, which  may  be  brought  about  by  a  great  diversity  of  causes. 
In  the  experiments  the  cause  probably  was  a  diseased  condition 
of  the  tissues  of  the  mother  herself.  She  had  been  subjected  to 
freezing  sufficiently  severe  to  prevent  the  proper  development  of 
the  pigments  and  some  of  the  ovarian  cells  presumably  suffered 
also.  It  will  be  observed  that  the  only  specimens  which  were 
affected  were  the  offspring  of  the  most  abnormal  female,  and  of 
them  only  four  out  of  forty- three  showed  any  change. 

The  same  interpretation  probably  applies  to  the  cases  in 

2  Standfuss,  M.,  Denks.  Schweiz.  naturf.  Ges.,  XXXVI,  1898,  p.  32. 
•Fischer,  E.,  Allg.  Ztschr.  f.  Entomologie,  Bd.  VI,  1901. 

4  Out  of  12  pupae  treated  8  died  and  of  the  4  survivors,  one  only  was  affected. 
See  M.  v.  Linden,  Archiv.  Rassen.  u.  Gesells.,  1904,  I. 


ADAPTATION  193 

Arctia  caja.  In  this  species  the  markings  are  well  known  to 
be  liable  to  great  variation.  As  Barrett  says,  even  in  nature 
individuals  are  rarely  quite  alike,  and  an  immense  number  of 
strange  forms  occur  in  collections.5  These  are  greatly  sought 
after  by  some  collectors,  especially  in  England,  where  they  fetch 
high  prices  at  auctions,  and  it  is  notorious  that  most  of  them  come 
from  Lancashire  and  the  West  Riding  of  Yorkshire.  It  is  com- 
monly supposed  that  the  breeders  of  that  district  subject  them  to 
abnormal  conditions,  and  especially  to  unnatural  feeding,  but 
I  know  no  clear  evidence  that  this  is  true.  From  whatever  cause 
it  is  certain  that  the  natural  pattern  is,  in  some  strains  at  all 
events,  very  easily  disturbed. 

The  elaborate  experiments  of  Schroder  with  Abraxas  grossu- 
lariata  are  difficult  to  follow  and  are  complicated  by  the  fact 
that  the  series  which  was  submitted  to  abnormal  temperatures 
was  derived  from  an  abnormal  original  pair.  From  the  evidence 
given  it  is  not  clear  to  me  whether  the  temperature  had  a  distinct 
effect.  This  insect,  like  A rctia  caja,  produces  an  immense  number 
of  variations  (especially  in  the  amount  of  the  black  pigment) 
and  as  most  of  these  are,  I  believe,  reared  in  domestication  for 
sale,  it  is  highly  probable  that  the  species  is  easily  influenced 
by  cultural  conditions. 

Schroder  describes  two  other  experiments  which  have  been 
accepted  by  Semon  and  other  supporters  of  the  view  that  acquired 
characters  are  transmitted.  In  the  first,  Phratora  vitellinae,  a 
phytophagous  beetle  living  on  the  undersides  of  leaves,  was  used. 
It  naturally  feeds  on  Salix  fragilis,  a  species  without  a  felt,  or 
tomentum,  on  the  underside  of  the  leaves.  Larvae  were  trans- 
ferred to  another  willow  (near  S.  viminalis)  which  has  the  under- 
sides of  the  leaves  felted.  The  larvae  took  readily  to  the  new 
food,  pushing  the  tomentum  before  them  as  they  gnawed  the 
leaves.  They  came  to  maturity  and  when  they  were  about  to 
lay  their  eggs  they  were  given  a  free  choice  between  S.  fragilis 
and  the  tomentose  species.  The  greater  number  of  ovipositions, 

6  For  illustrations  see  Oberthur's  Etudes  d'Entom.,  1896,  where  many  of  these 
curious  aberrations  are  represented;  also  Barrett,  Lepid.  Brit.  Islands,  II,  pp.  71 
and  72. 

14 


194  PROBLEMS  OF  GENETICS 

£19,  took  place  on  fragilis,  and  there  were  127  on  the  tohientose 
bush,  which  we  are  told  was  six  times  as  large  as  the  fragilis. 
The  larvae  from  fragilis  were  next  put  on  the  tomentose  species 
and  reared  on  it.  When  they  became  imagines  they  were  simi- 
larly given  their  choice,  with  the  result  that  there  were  104 
ovipositions  on  the  tomentose  species  and  only  83  on  fragilis. 
In  the  next  generations  there  were  48  ovipositions  on  the  tomen- 
tose and  ii  on  fragilis.  Finally  the  fourth  generation  made 
15  ovipositions  on  the  tomentose  and  none  on  fragilis. 

The  difficulty  about  such  experiments  is  obviously  that  one 
has  no  assurance  that  the  change  of  instinct,  in  so  far  as  there 
is  any,  may  not  be  a  mere  consequence  of  the  captivity.  It 
must,  besides,  be  extremely  difficult  to  arrange  the  experiment  so 
that  there  is  really  an  equal  choice  between  the  two  bushes,  when 
one  stands  beside  the  other.  Przibram,  in  quoting  this  case, 
considers  that  as  the  tomentose  bush  was  about  six  times  as 
large  as  the  fragilis ,  some  indication  of  the  relative  attractiveness 
of  the  two  may  be  obtained  by  dividing  the  ovipositions  on  the 
larger  bush  by  six,  but  I  imagine  the  matter  must  be  much  more 
complex. 

Schroder's  second  example  is  not  more  convincing,  in  my 
opinion,  though  Semon  regards  it  as  one  of  the  most  important 
pieces  of  evidence.  It  concerns  a  leaf-rolling  moth,  Gracilaria 
stigmatella,  the  larva  of  which  is  said  normally  to  make  its  house 
by  bending  over  the  tips  of  the  sallow  leaves  on  which  it  feeds. 
Schroder  placed  larvae  on  leaves  from  which  the  tips  had  been 
cut,  and  these  larvae  made  their  houses  by  rolling  over  the  sides 
of  the  leaves.  Their  offspring  were  again  fed  on  leaves  without 
tips,  and  as  before,  they  rolled  in  the  leaf-margins  either  on  one 
side  or  both.  The  offspring  of  this  second  generation  were  then 
fed  on  entire  leaves.  There  were  19  houses  made  by  these  (?I9) 
larvae,  and  of  them  15  were  normal,  made  by  folding  down  the 
tips  of  the  leaves,  while  4  were  abnormal,  made  by  rolling  in  the 
leaf-margins.  Schroder  says  that  in  nature  he  has  only  twice 
seen  abnormal  houses ;  but  it  is  clearly  essential  not  only  that  the 
frequency  of  such  variability  in  nature  should  be  thoroughly 
examined,  but  also  that  we  should  know  whether  when  the  species 


ADAPTATION  195 

is  bred  in  captivity  these  irregularities  of  behaviour  do  or  do  not 
occur  when  the  larvae  are  fed  on  uninjured  leaves. 

The  famous  case  of  Schiibeler's  wheat  is  revived  by  Semon. 
The  story  will  be  familiar  to  most  readers  of  the  literature  of  the 
subject.  Briefly  it  is  that  annuals,  especially  wheat  and  maize, 
raised  from  seed  in  Central  Europe  take  more  time  in  coming 
to  maturity  and  ripening  than  similar  plants  raised  in  Norway, 
where  the  summer  days  are  much  longer.  The  received  account 
is  that  he  imported  seed  especially  of  maize  and  of  wheat  from 
Central  Europe  to  Norway  and  found  that  in  successive  years 
the  period  of  growth  and  ripening  was  increasingly  reduced. 
After  two  generations  seed  of  the  accelerated  wheat  was  sent 
back  to  Breslau  where  it  was  grown,  and  was  found  to  ripen  rather 
more  slowly  than  in  Norway,  but  much  more  quickly  than  the 
original  stock  had  done.  The  facts  recorded  by  Schlibeler6  are 
that  he  received  seed  from  Eldena,  which  is  on  the  Baltic  near 
Greifswald.  The  variety  is  described  as  "  100  tdgiger  Sommer 
Weizen"  but  no  more  exact  record  of  its  behaviour  in  Germany 
is  given.  This  wheat,  grown  at  Christiania  in  1857,  took  103 
days  to  harvest.  Its  seed  was  again  grown  in  Christiania  in  1858, 
and  took  93  days,  and  sown  again  in  1859  it  took  only  75  days,  28 
days  less  than  in  the  first  year  of  cultivation  in  Norway.  Seed  of 
the  1858  crop  was  sent  to  Breslau,  and  grown  there  by  Roedelius 
in  1 859;  it  took  80  days.  Evidently  before  such  a  record  can  be 
used  as  proving  an  inheritance  of  acquired  characters  numbers  of 
particulars  should  be  forthcoming.  The  view  that  Johannsen 
has  taken  is  that  the  result  was  probably  due  to  unconscious 
selection  of  the  earlier  individuals  among  a  population  consisting 
of  many  types  of  various  compositions.  Some  effect  may  no 
doubt  be  ascribed  to  that  cause,  but  I  cannot  think  that  alone 
it  would  account  for  the  results.  My  impression  is  rather  that 
they  were  produced  by  differences  in  the  cultivation  and  especially 
in  the  seasons.  Research  of  an  elaborate  character  would  be 
necessary  in  order  to  eliminate  the  various  sources  of  error,  and 
nothing  of  the  kind  has  been  done;  nor  does  Semon  allude  to  these 
difficulties  in  prominently  adducing  Schiibeler's  evidence.  A 

6  Schiibeler,  F.  C.,  Die  Culturpflanzen  Norwegens,  1862,  especially  pp.  24  and  28. 


196  PROBLEMS  OF  GENETICS 

difference  of  even  three  weeks  in  time  of  harvesting  may  easily 
be  due  to  variation  in  the  season.  It  would  in  any  case  be  dif- 
ficult to  analyse  the  meteorological  conditions,  and  to  decide  how 
much  effect  in  postponing  or  accelerating  the  harvest  might  be 
due  to  cold  days,  to  cloudy  days,  to  wet  weather,  to  fluctuations 
in  average  temperature,  to  hot  days,  and  other  such  incidents 
occurring  at  the  different  periods  of  growth,  even  if  they  were 
specially  watched  while  the  experiments  were  in  progress,  and 
at  this  distance  of  time  such  analysis  is  practically  impossible. 
Without  careful  simultaneous  control-experiments  this  evi- 
dence is  almost  worthless.  The  director  of  the  Meteorological 
Office7  has,  however,  kindly  sent  me  some  details  of  the  weather 
at  Breslau  from  1857  to  1860,  and  I  notice  that  as  a  matter  of 
fact  July,  1859,  was  an  exceptionally  hot  month,  having  an  average 
0/2.67°  C-  above  the  mean  for  the  twenty  years  1848-1867.  June 
in  that  year  was  slightly  (0.31°  C.)  below  the  mean  and  May 
slightly  above  it  (0.18°  C.).  August  was  also  abnormally  hot, 
2.35°  C.  above  the  average.  The  Breslau  wheat  was  sown  on 
May  ip  and  harvested  on  August  6.  There  was  a  cold  spell  from 
May  II  to  14,  which  this  wheat  escaped,  as  it  was  sown  on  May 
19.  In  the  other  years  the  cold  spell  came  much  later.  These 
elements  of  the  weather  may  possibly  have  done  something  to 
hurry  the  ripening  in  1859.  It  is  unfortunate  that  we  are  not 
told  how  long  similar  wheat  from  Breslau  seed  took  to  ripen  in 
that  year. 

As  regards  the  Norway  cultivations  we  have  the  average 
monthly  temperatures  recorded  by  Schiibeler,  though  he  does 
not  discuss  them  in  connection  with  this  special  problem.  It  is 
quite  clear  that  1857,  in  which  the  period  was  103  days,  was  an 
exceptionally  cold  summer,  especially  as  regards  the  months  of 
June  and  July,  but  though  there  was,  so  far  as  the  temperature 

1 1  am  obliged  to  him  and  to  Dr.  E.  Gold  for  much  trouble  taken  to  answer 
my  questions.  Some  idea  of  the  kind  of  weather  indicated  by  an  average  of  2.7°6 
C.  above  the  mean  may  be  got  from  a  comparison  with  the  year  1911,  which  most 
people  will  remember  as  one  of  the  hottest  summers  they  have  known.  The 
July  of  that  year  was  in  east  and  southeast  England  about  4°  F.  above  the  mean 
but  2.67  C.  means  about  4.8°  F.  above  the  mean.  At  Greenwich  July,  1859,  was 
about  6.5°  F.  above  the  average. 


ADAPTATION  197 

records  go,  no  great  difference  between  1858  and  1859,  the  year 
1859,  in  which  the  period  of  ripening  was  the  shortest,  was  some- 
what colder  in  Norway  than  1858.  But  we  have  the  further 
difficulty  that  there  were  ten  days  difference  in  sowing,  for  in 
1858  the  sowing  was  made  on  May  14,  and  in  1859  on  May  24. 
With  all  these  possibilities  uncontrolled,  and  indeed  unconsidered, 
I  am  surprised  that  Semon  should  claim  these  experiments  as  one 
of  the  chief  supports  for  his  views. 

Schiibeler's  other  allegations  respecting  the  influence  of 
climate  on  plants  grown  in  various  places  and  especially  at  dif- 
ferent elevations  in  Norway  have  been  destructively  criticised  by 
Wille8  to  whose  paper  readers  interested  in  the  subject  should 
refer. 

Before  the  appearance  of  Wille's  criticisms  Wettstein9  made 
a  favourable  reference  to  Schiibeler's  work,  accepting  his  con- 
clusion. He  states  also  that  he  has  himself  made  analogous 
experiments  with  flax,  finding  that  the  length  of  the  period  of 
development  and  a  series  of  morphological  characters  show  an 
adaptation  to  local  conditions,  and  that  on  transference  of  seed 
to  other  conditions  the  previous  effects  are  maintained.  No 
details,  however,  are  given,  and  I  do  not  know  if  anything  more 
on  the  subject  has  appeared  since.  The  other  examples  cited 
by  Wettstein,  such  as  the  observations  of  Cieslar  on  forest-trees 
and  those  of  Jakowatz  on  gentians  seem  to  me  open  to  all  the 
usual  objections  applicable  to  evidence  of  this  kind.  Such  work, 
to  be  of  any  value  for  the  purpose  to  which  it  is  applied,  must  be 
preceded  by  a  study  of  the  normal  heredity  and  of  the  variations 
of  the  species. 

Most  of  the  recent  writers  (Semon,  Przibram,  etc.)  on  the 
inheritance  of  acquired  characters  accept  the  story  of  Brown- 
S6quard's  guinea  pigs,  which  are  said  to  have  inherited  a  liability 
to  peculiar  epileptiform  attacks  induced  in  their  parents  by  various 
nervous  lesions. 

The  question  has  been  often  debated  and  several  observers 
have  repeated  the  experiments  with  varying  results,  some  failing 

s  Wille,  N.,  BioL  Cbltt.,  XXV,  1905,  p.  521. 

9  Wettstein,  R.  von,  Der  Neo-marckismus  u.  seine  Beziehungen  zum  Danvinis- 
mus,  Jena,  1903- 


198  PROBLEMS  OF  GENETICS 

to  confirm  Brown-S6quard,  others  finding  evidence  which  in 
various  degrees  supported  his  conclusions.  Recently  a  new  and 
especially  valuable  paper  has  been  published  by  Mr.  T.  Graham 
Brown10  which  goes  far  towards  settling  this  outstanding  question. 
He  states  that  "the  Brown-S6quard  phenomenon  is  nothing  more 
or  less  than  a  specific  instance  of  the  scratch-reflex,"  and  it  is 
due  to  a  raised  excitability  of  the  mechanism  of  this  reflex.  This 
raised  excitability  is  the  character  acquired  as  a  consequence, 
for  instance,  of  the  removal  of  part  of  one  great  sciatic  nerve. 
The  nature  of  this  raised  excitability  and  its  causation  are  dis- 
cussed and  elucidated,  but  this  part  of  the  work  is  not  essential 
to  the  present  consideration.  Mr.  Graham  Brown  in  his  summary 
of  conclusions  remarks  that  it  is  very  difficult  to  see  how  this  con- 
dition of  raised  excitability  can  be  transmitted  to  the  offspring, 
and  this  comment  which  might  be  made  in  reference  to  any  of 
the  alleged  cases  certainly  applies  with  special  cogency  to  the 
present  example. 

He  then  calls  special  attention  to  three  observations: 

1.  That  guinea  pigs  which  had  a  "  trophic  "  change  in  the 
foot,  as  a  result  of  division  of  the  great  sciatic  nerve,  have  re- 
peatedly been  seen  to  nibble  the  feet  of  other  guinea  pigs  which 
had  this  change  in  the  foot  from  the  same  causes. 

2.  That  accidental  injury  to  the  toes  may  be  followed  by  the 
Brown-S6quard  phenomenon  in  an  otherwise  normal  animal. 

3.  That  in  several  instances  the  young  of  guinea  pigs  which 
exhibited  the  phenomenon  have  been  noticed  to  have  one  or  more 
toes  eaten  off  by  the  mother. 

Brown-Sequard  noticed  that  almost  all  his  animals  in  which 
the  great  sciatic  was  divided  acquired  the  "  epilepsy  "  and 
nibbled  those  parts  of  their  feet  in  which  sensation  had  been  lost. 
Of  the  offspring  of  such  animals  he  found  that  a  very  small  pro- 
portion exhibited  a  malformation  of  the  feet,  and  of  these  some 
showed  the  "  epilepsy."  The  proportion  which  showed  the 
"  epilepsy  "  was  one  to  two  per  cent,  of  the  offspring. 

Morgan11  is  quoted  by  Graham  Brown  as  having  suggested 

10  T.  Graham  Brown,  Proc.  Roy.  Soc.,  1912,  vol.  84,  B,  p.  555.     This  paper  gives 
full  reference  to  the  previous  literature  of  the  subject. 

11  Morgan,  T.  H.,  Evolution  and  Adaptation,  New  York,  1903. 


ADAPTATION  199 

that  the  loss  of  toes  in  the  offspring  may  have  been  due  to  mu- 
tilation by  the  mother,  following  his  experience  in  a  case  in  which 
the  tails  of  mice  in  succeeding  litters  were  thus  devoured,  and 
there  can  be  little  doubt  that  in  this  suggestion  lies  the  clue  to 
the  explanation  of  the  whole  mystery.  Graham  Brown  concludes 
that  it  may  be  supposed  with  every  degree  of  probability  that 
the  "  transmission  "  was  due  to  injuries  inflicted  upon  the  young 
by  their  parents.  With  this  conclusion  most  people  will  now  be 
disposed  to  agree,  and  we  may  hope  that  we  shall  hear  the  last 
of  this  curious  myth  —  to  the  elucidation  of  which  a  vast  quan- 
tity of  research  has  been  devoted. 

The  series  of  experiments  made  by  Kammerer  with  various 
Amphibia  have  attracted  much  attention  and  have  been  ac- 
claimed by  Semon  and  other  believers  in  the  transmission  of 
acquired  characters  as  giving  proof  of  the  truth  of  their  views. 
With  respect  to  these  observations  the  chief  comment  to  be  made 
is  that  they  are  as  yet  unconfirmed.  Many  of  the  results  that 
are  described,  it  is  scarcely  necessary  to  say,  will  strike  most 
readers  as  very  improbable;  but  coming  from  a  man  of  Dr. 
Kammerer 's  wide  experience,  and  accepted  as  they  are  by  Dr. 
Przibram,  under  whose  auspices  the  work  was  done  in  the  Bi- 
ologische  Vesuchsanstalt  at  Vienna,  the  published  accounts  are 
worthy  of  the  most  respectful  attention. 

The  evidence  relates  chiefly  to  three  distinct  groups  of  oc- 
currences : 

1.  Modification   in  Alytes  obstetricans,  the    Midwife  Toad, 
affecting  both  the  structure  and  the  mode  of  reproduction,  in- 
duced by  compulsory  change  of  habits. 

2.  Modification  in  the  mode  of  reproduction  of  Salamandra 
atra  and  maculosa  induced  by  compulsory  change  of  habits. 

3.  Modification  in  the  colour  of  Salamandra  maculosa  induced 
by  change  in  the  colour  of  the  soil  on  which  the  animals  were  kept. 

I.  I  will  take  first  the  case  of  Alytes,12  because  it  is  the  most 

12  Kammerer's  chief  paper  on  this  subject  is  in  Arch.  f.  Entwm.,  1909,  XXVIII, 
p.  447,  and  it  is  to  this  that  the  paginal  references  in  the  present  text  relate.  His 
previous  paper  appeared,  ibid.,  1906,  XXII,  p.  48.  An  account  of  his  further  ex- 
periments with  Alytes  is  given  in  Natur,  1909-10,  Heft  6,  p.  95. 


200  PROBLEMS  OF  GENETICS 

definite  example,  and  because  it  is  the  case  which  most  readily 
admits  of  repetition  and  verification. 

The  habits  of  Alytes  obstetricans  are  well  known.  The 
animals  copulate  on  land.  As  the  strings  of  eggs  leave  the 
female  they  are  entangled  by  the  hind  legs  of  the  male,  and  being 
adhesive  they  stick  to  him  and  undergo  their  development  at- 
tached to  his  back  and  legs.  The  number  of  eggs  varies  from  18 
to  86,  a  number  much  smaller  than  is  usual  in  toads  and  frogs 
which  lay  their  eggs  in  water.  The  eggs  are  large  and  full  of  yolk. 

There  are  two  breeding  seasons,  one  about  April  and  the  other 
about  September,  and  a  winter  hibernation.  Not  only  animals 
brought  in  from  outside,  but  their  offspring  reared  in  domestica- 
tion maintain  these  normal  habits  in  confinement,  if  the  temper- 
ature does  not  exceed  17°  C.  (pp.  499  and  534). 

If,  however,  the  temperature  be  artificially  raised  and  kept 
at  25-30°  C.,  the  males  do  not  attach  the  eggs  to  themselves 
when  spawning  occurs  on  land  but  let  them  lie.  The  adhesion 
of  the  eggs  is  said  to  be  hindered  by  the  comparatively  rapid 
drying  of  their  surfaces. 

More  usually  in  the  high  temperatures  the  animals  take  to 
the  water  and  copulate  there.  The  eggs  are  ejected  into  the 
water,  and  as  their  gelatinous  coverings  immediately  swell  up, 
they  do  not  stick  to  the  males. 

The  offspring  thus  derived  from  the  parents  subjected  to 
heat  for  one  breeding-period  only,  whether  they  were  laid  in  water 
or  on  land,  did  not  show  departures  from  the  normal  type. 

Kammerer  states  next,  however,  that  in  subsequent  breeding- 
periods  the  same  parents  frequently  take  to  the  water  to  breed, 
though  they  have  become  quite  accustomed  to  the  heated 
chamber;  and  furthermore  that  if  such  animals,  having  thus  lost 
their  instinct  to  brood  their  young,  be  transferred  to  ordinary 
temperatures  they  do  not  readily  reassume  their  normal  habits, 
but  for  several  breeding  seasons  —  at  least  four  —  will  take  to 
the  water.  These  parents  lay  from  90  to  115  eggs,  which  are 
small  and  contain  little  yolk,  and  the  larvae,  on  hatching,  breathe 
with  their  embryonic  gills  until  they  are  absorbed  instead  of 
being  broken  off  as  normally. 


ADAPTATION  201 

The  offspring  thus  abnormally  developed  when  they  mature 
are  said  never  to  brood  their  eggs.  If  they  are  derived  from 
the  earlier  spawnings  of  their  parents,  before,  that  is  to  say,  the 
parents  had  been  submitted  to  the  changed  conditions  long  enough 
to  transmit  their  effects,  they  lay  on  land ;  but  if  they  are  derived 
from  the  later  spawnings,  they  lay  in  the  water.  These  changes 
of  habit  are  manifested  without  the  continued  application  of 
the  abnormal  experimental  conditions,  and,  as  I  understand  the 
account,  in  normal  conditions  of  temperature. 

If  the  abnormal  experimental  conditions  are  continued,  the 
toads  always  lay  in  water,  and  their  eggs  become  progressively 
smaller  and  more  numerous.  The  larvae  in  the  fourth  generation 
acquire  three  pairs  of  gills  instead  of  one  pair,  and  are  in  other 
respects  also  different  from  the  normal  form. 

Respecting  the  Alytes  bred  in  this  way  Kammerer  makes  the 
very  striking  statement  that  the  males  in  the  third  generation 
(p-  535)  have  roughened  swellings  on  their  thumbs  and  that  in  the 
fourth  generation  (pp.  516  and  535)  these  swellings  develop  black 
pigment.  Together  with  the  appearance  of  this  secondary  sexual 
character  there  is  hypertrophy  of  the  muscles  of  the  fore-arm. 
To  my  mind  this  is  the  critical  observation.  If  it  can  be  sub- 
stantiated it  would  go  far  towards  proving  Kammerer's  case. 
Alytes,  among  toads  and  frogs,  is  peculiar  in  that  the  males  do 
not  develop  these  lumps  in  the  breeding  season,  and  the  fact 
may  no  doubt  be  taken  to  be  correlated  with  the  breeding  habits, 
copulation  occurring  on  land  and  not  in  water  as  is  usual  with 
Batrachians.  It  is  to  be  expressly  noticed  that  these  lumps  on 
the  thumbs  or  arms  of  male  toads  and  frogs  are  not  merely  pig- 
mented  swellings,  but  are  pads  bearing  numerous  minute  horny 
black  spines,  which  are  used  in  holding  the  females  in  the  water. 
The  figures  which  Kammerer  gives  (Taf.  XVI,  figs.  26  and  260) 
are  quite  inadequate,  and  as  they  merely  indicate  a  dark  patch 
on  the  thumbs  it  is  not  possible  to  form  any  opinion  as  to  the 
nature  of  the  structure  they  represent. 

The  systematists  who  have  made  a  special  sudy  of  Batrachia 
appear  to  be  agreed  that  Alytes  in  nature  does  not  have  these 
structures;  and  when  individuals  possessing  them  can  be  pro- 


202  PROBLEMS  OF  GENETICS 

duced  for  inspection  it  will,  I  think  be  time  to  examine  the  evi- 
dence for  the  inheritance  of  acquired  characters  more  seriously. 
I  wrote  to  Dr.  Kammerer  in  July,  1910,  asking  him  for  the  loan 
of  such  a  specimen13  and  on  visiting  the  Biologische  Versuchsan- 
stalt  in  September  of  the  same  year  I  made  the  same  request,  but 
hitherto  none  has  been  produced.  In  matters  of  this  kind  much 
generally  depends  on  interpretations  made  at  the  time  of  ob- 
servation; here,  however,  is  an  example  which  could  readily 
be  attested  by  preserved  material.  I  notice  with  some  surprise 
that  in  a  later  publication  on  the  same  subject  no  reference  to  the 
development  of  these  structures  is  made  (see  below) . 

The  statements  here  given  represent  but  a  small  part  of 
Kammerer 's  papers  on  the  subject.  He  gives  much  further 
information  as  to  the  course  of  the  experiments,  especially  in 
regard  to  the  fate  of  the  eggs  laid  on  land  and  the  aberrations 
induced  in  them  by  treatment.  The  ramifications  of  the  ex- 
periments are,  however,  very  difficult  to  follow,  and  as  I  am  not 
sure  that  I  have  always  understood  them  I  must  refer  the  reader 
to  the  original. 

More  recently  Kammerer  has  published14  a  most  curious 
account  of  experiments  in  crossing  his  modified  and  abnormal 
Alytes,  derived  from  the  water-eggs,  with  normal  individuals. 

In  the  first  case  the  cross  was  made  between  a  normal  female 
and  an  abnormal  male.  The  offspring  were  normal  in  their 
habits.  In  the  next  generation  bred  from  these  almost  exactly 
a  quarter  showed  the  abnormal  instinct. 

The  reciprocal  cross  was  made  between  an  abnormal  female 
and  a  normal  male.  In  this  case  the  offspring  were  abnormal  in 
their  behaviour;  but  the  second  generation  bred  from  them 
showed  three  quarters  abnormal  and  one  quarter  normal. 

Certain  details  as  to  numbers  and  sexes  of  the  various  families 
bred  in  the  course  of  this  amazing  experiment  are  given  in  a 

13  In  reply  to  my  letter  Dr.  Kammerer  who  was  then  away  from  home  very 
kindly  replied  that  he  was  not  quite  sure  whether  he  had  killed  specimens  of  Alytes 
with  "  Brunftschwielen "  or  whether  he  only  had  living  males  of  the  fourth  genera- 
tion, but  that  he  would  send  illustrative  material. 

14  Kammerer,  P.,  Natur,  12  December,  1909,  Heft  6,  p.  95,  repeated  in  12 
Flugschrift  d.  Deutsch  Ges.  f.  Ziichtungskunde,  Berlin,  1910. 


ADAPTATION  203 

subsequent  publication.15  This  later  paper  goes  somewhat  fully 
into  the  question  of  the  difference  in  behaviour  between  the 
normal  and  modified  individuals,  describing  the  ways  in  which 
the  males  and  females  possessing  the  acquired  character  could  be 
recognised  from  the  males  and  females  which  were  normal,  but 
in  this  account  I  find  no  reference  to  the  development  of  the 
"  Brunftschwielen  "  —  the  horny  pads  on  the  hands  of  the  males. 
As  these  structures  would  be  of  special  value  in  such  a  diagnosis 
the  omission  of  any  allusion  to  them  calls  for  explanation. 
Kammerer  claims  the  evidence  as  proof  of  Mendelian  segregation 
in  regard  to  an  acquired  character,  the  first  example  recorded. 
Pending  a  repetition  of  the  experiments  there  is  no  more  to  be 
said. 

2 .  The  Mode  of  Reproduction  of  Salamandra  atra  and  maculosa.1* 
—  Salamandra  maculosa,  the  common  lowland  form,  with  yellow 
bands  or  spots,  deposits  its  young  in  water,  generally  as  gill- 
bearing  tadpoles,  with  a  wide,  swimming  tail,  though  occasionally 
they  are  born  still  enclosed  in  the  egg-capsule  out  of  which  they 
soon  hatch.  Spawning  extends  over  a  considerable  period, 
often  many  weeks,  and  during  the  season  one  female  may  bear 
more  than  50  young. 

S.  atra,  the  black  Alpine  form,  produces  its  young  on  land. 
They  are  born  without  gills,  ready  to  breathe  air,  and  with  the 
rounded  tail  of  the  adult.  These  differences  may,  as  Kammerer 
says,  naturally  be  regarded  as  adaptations  to  the  Alpine  condi- 
tions. Moreover,  the  female  bears  only  two  young  in  a  season, 
and  this  reduction  in  the  number  must  be  taken  to  be  a  conse- 
quence or  condition  of  viviparity.  There  are  many  eggs  in  the 
ovary,  but  all  except  the  two  which  are  destined  to  develop 
degenerate  and  form  a  yolk-material  on  which  these  two  sur- 
vivors feed. 

Kammerer  gives  a  long  account  of  the  various  conditions  to 
which  he  subjected  both  species.  The  treatment  was  complicated 

16  Festschrift  zum  Andenken  an  Gregor  Mendel,  being  vol.  XLIX  of  the  Ver h. 
Naturf.  Ver.  in  Brunn,  1911,  p.  98. 

16  Kammerer's  chief  papers  on  this  subject  are  Archiv  fur  Entwm.,  XVII, 
1904,  and  ibid.,  XXV,  1907.  An  epitome  of  results  is  also  given  by  him  in  12 
Flugschrift  d.  Deutsch.  Ges.  f.  Ziichtungskunde,  Berlin,  1910. 


204  PROBLEMS  OF  GENETICS 

in  many  ways,  but  the  essential  statements  are,  as  regards 
S.  maculosa,  that  when  no  water  was  provided  in  which  the  young 
might  be  born,  they  were  dropped  on  land,  larger  and  in  a  later 
stage  of  development  and  of  a  darker  colour  than  is  normal;  that 
the  larvae  so  born  gradually  diminished  in  number  until  only 
two  were  deposited  in  each  breeding-period;  that  dissection 
showed  that  the  other  ova  degenerated  to  form  a  yolk-material. 
The  larvae  so  produced  reached  maturity.  The  summary  of 
results  describes  their  behaviour,  stating  that  they  produced: 

(a)  In  water,  either  (i)  very  advanced,  large-headed  larvae 
45  mm.  long  (instead  of  25-30  mm.)  with  gills  already  reduced, 
which  had  awkward,  embryo-like  movements,  and  in  some  few 
days    metamorphosed  into  small  perfect  salamanders;  or   (2) 
moderately  advanced,  properly  proportioned  larvae,  40-41  mm. 
long,  provided  with  large  gills  of  (at  first)  intrauterine  character, 
which  were  reduced  during  aquatic  life. 

(b)  On  land,  small  (26  mm.  long)  larvae  with  rudimentary 
gills,  having  the  body  rounded  instead  of  being  flattened  from 
above  downwards,  and  an  elongated  narrow  head,  which  were 
unable  to  live  in  deep  water.     These  larvae  changed  to  the  sala- 
mander colour  in  10-12  days,  and  after  four  weeks  metamor- 
phosed into  salamanders  29  mm.  long. 

(c)  In  the  foregoing  cases  the  experimental  conditions  were 
not  continued,  or  in  other  words,  basins  of  water  were  provided 
in  which  they  could  spawn.     But  if  the  experimental  conditions 
are   continued,   these   Salamandra  maculosa  which   were   born 
newt-like  (viz.,  not  in  a  larval  condition),  are  themselves  newt- 
bearing  from  the  first  time  they  give  birth,  using  the  dry  land, 
and  bringing  forth  only  two  young,  the  normal  number  for  the 
births  of  5.  atra.     These  young  are  40-41  mm.  long,  and  are 
dark-coloured,  resembling  greatly  the  normal  new-born  S.  atra. 

This  epitome  of  the  observations  illustrating  the  inheritance 
of  acquired  characters  has  been  very  widely  quoted,  and  may 
not  unnaturally  be  taken  to  summarize  a  wide  experience  of 
the  modified  animals.  Reference  to  the  details  given  in  the 
same  paper  shows  that,  as  alleged,  each  of  the  four  types  of  be- 
haviour enumerated  was  witnessed  once  only  in  the  case  of  each 


ADAPTATION  205 

of  four  females,  no  two  agreeing  with  each  other.  As  to  the 
number  of  the  males  or  their  habits  nothing  is  said.  The  first 
female,  a  (i),  bore  five  young;  the  second,  a  (2),  bore  two,  of 
which  one  was  a  partial  albino;  the  third,  &,  produced  four  young; 
and  the  fourth,  c,  two  as  already  stated. 

In  the  case  of  c  the  details  show  that  the  female  gave  birth 
immediately  after  being  transferred  from  the  open-air  terrarium 
to  one  indoors,  which  contained  no  basin  of  water.  This  is  the 
example  of  the  consequences  which  follow  on  a  continuance  of 
the  experimental  conditions.17 

As  regards  S.  atra  the  converse  is  reported.  Various  means 
were  used  to  induce  them  to  eject  their  young  prematurely  in 
water,  such  as  massaging  the  sides  of  the  mothers,  or  raising 
the  temperature  to  25°  or  30°  C.,  with  various  degrees  of  success. 
But  afterwards  it  was  found  that  specimens  collected  wild  at  an 
elevation  of  about  1,000  metres  responded  to  much  simpler 
treatment,  and  gave  birth  prematurely  in  water  when  they  were 
kept  in  a  large  shallow  basin  of  water  not  so  deep  but  that  they 
could  everywhere  touch  the  bottom  with  their  feet  and  keep  their 
heads  above  the  surface.  With  specimens  collected  at  higher 
elevations  this  treatment  was  inoperative,  and  the  suggestion  is 
made  that  S.  atra  at  the  lower  confines  of  its  habitat  partakes 
more  of  the  nature  of  maculosa  than  do  the  individuals  from 
greater  heights;  for  Kammerer  argues  that  pools  suitable  for 
breeding  must  be  more  uncommon  at  those  elevations  than  they 
are  lower  down. 

In  the  earlier  paper18  Kammerer  states  that  newly  caught 
females  of  S.  atra  often  give  birth  in  the  water,  and  show  an 
undoubted  preference  for  doing  so.  He  describes  also  how  he 
once  saw  several  females,  wild  in  their  natural  habitat,  lay  their 
young  in  a  rain-puddle  at  1,800  metres  elevation,  but  the  larvae 
thus  born  were  fully  formed. 

When  the  deposition  of  the  young  as  larvae  has  become 

17  "  Bei  Fortdauer  der  Versuchsbedingungen  sind  ah  Vollmolche  geborene  Sala- 
mandra  maculosa  gleich  bei  der  ersten  Geburt  abermals  voll  molchgebarend,  benutzen 
zum  Geburtsakt  das  trockene  Land,  und  zwar  unter  Erreichung  der  (bei  Salamandra 
atra  normalen)  Embryonen-Zweizahl,"  Kammerer,  1907,  p.  49. 

18 1904,  p.  56. 


206  PROBLEMS  OF  GENETICS 

"  habitual  "19  with  S.  atra,  three  to  nine  larvae  may  be  produced 
at  one  spawning  period,  from  35  to  45  mm.  long,  with  gills  at 
most  8  mm.  long,  and  a  tail-fin  2-3  mm.  broad.  Such  larvae 
are  generally  coffee-brown,  or  grey  (instead  of  black),  and  show 
other  minor  differences. 

The  summary  states  that  when  grown  to  maturity  they  be- 
come in  their  turn  larva-bearing,  and  go  into  the  water  to  bring 
forth.  Their  young  are  more  than  two  (3  to  5  being  the  numbers 
observed)  with  a  length  of  33-40  mm.  or  of  21-23  mm-  at  birth. 
They  are  light  grey,  spotted  (mottled  with  lighter  and  darker 
colour),  have  relatively  short  gills  (8  to  9  mm.  at  most)  and  a 
broad  tail-fin  (3  mm.  wide).  At  metamorphosis  they  are  rela- 
tively long  (44  mm.)  and  one  of  them  had  some  yellow  pigment. 

Here  again  this  summary  is,  as  a  matter  of  fact,  describing 
the  behaviour  of  two  mothers,  of  which  one  produced  three,  and 
the  other  five  young. 

To  my  mind  these  experiments  suggest  that  the  reproductive 
habits  of  both  species,  if  closely  observed,  will  be  found  to  be 
subject  to  considerable  variation,  and  I  think  it  not  impossible 
that  each  species  is,  especially  in  confinement,  capable  of  being  a 
good  deal  deflected  from  its  normal  behaviour.  Moreover,  there 
seems  to  me  no  great  improbability  in  the  idea  that  there  is  an 
interdependence  between  the  number  of  young  and  the  stage  of 
maturity  in  which  they  are  born.  But,  at  the  same  time,  the  case 
as  told  by  Kammerer  strikes  me  as  proving  too  much.  If  each 
species  is  so  sensitive  to  conditions  that  the  normal  procedure 
is  gravely  modified  in  one  generation,  and  if  that  modification 
can  reappear  in  a  pronounced  form  in  the  next  generation  without 
a  renewal*  of  the  disturbing  conditions,  it  becomes  extremely 
difficult  to  understand  how  the  regularity  which  each  species  is 
believed  to  display  in  nature  can  be  maintained.  Surely  both 
species  might  be  expected  to  be  in  confusion.  From  a  passage 
in  Kammerer's  earlier  paper  (1904,  p.  55)  on  the  subject,  I  infer 
that  he  also  would  expect  considerable  irregularity  in  the  natural 
behaviour,  but  that  he  has  not  investigated  the  point.20 

19  Throughout  Kammerer's  papers  this  is  used  almost  as  a  technical  term. 
It  means,  I  presume,  that  the  feature  was  manifested  more  than  once. 

20  It  should  be  stated  that  the  papers  contain  a  quantity  of  detail,  especially 


ADAPTATION  207 

3.  Modification  of  the  Colour  of  Salamandra  maculosa  induced 
by  Change  in  the  Colour  of  the  Soil  on  which  the  Animals  were 
kept. — Kammerer  speaks  of  this  as  the  most  convincing  of  all 
his  experiments  on  the  transmission  of  acquired  characters.  So 
far,  however,  no  full  account  of  them  has  been  published.21  The 
statement  is  that  when  salamanders  are  kept  in  yellow  surround- 
ings the  yellow  markings  gradually  in  the  course  of  years  increase 
in  amount  relatively  to  the  black  ground  colour.  Conversely  by 
keeping  the  animals  on  black  garden  soil,  the  yellow  may  be 
greatly  diminished  in  quantity  until  it  largely  disappears.  (The 
account  in  Natur  adds  that  very  moist  conditions  also  favour  the 
increase  of  yellow,  and  that  with  less  moist  conditions  the  yellow 
diminishes.)  From  each  kind,  the  (induced)  yellower  and  the 
(induced)  blacker,  a  second  generation  was  raised,  on  soil  of 
neutral  colour,  and  each  family  was  later  divided  into  two  parts, 
half  being  put  on  black  and  half  on  yellow  ground. 

As  regards  the  offspring  of  those  which  had  lived  on  black 
soil  no  positive  result  had  been  reached  up  to  the  date  of  publica- 
tion, but  it  is  stated  that  these  young  resembled  their  parents 
in  having  the  yellow  distributed  in  irregular  spots. 

As  regards  the  offspring  of  those  which  had  lived  on  yellow 
soil  the  account  follows  up  the  story  of  that  part  of  the  offspring 
which  were  put  on  yellow  soil  again.  It  is  stated  that  these,  though 
derived  from  parents  with  irregular  spots,  developed  the  yellow 
as  longitudinal  bands. 

This  account  is  given  with  slight  differences  of  expression  in 
the  three  places  to  which  I  have  referred.  On  returning  from 
Vienna  in  1910  I  consulted  Mr.  G.  A.  Boulenger  in  reference  to 
the  subject,  and  he  very  kindly  showed  me  the  fine  series  from 
many  localities  in  the  British  Museum,  and  pointed  out  that  in 
nature  the  colour- varieties  can  be  grouped  into  two  distinct  types, 

descriptive  of  the  state  of  the  larvae,  which  I  have  not  attempted  to  represent,  but 
the  account  here  given  contains  all  that  seemed  essential  to  an  understanding  of 
the  more  important  features  of  the  account. 

21  The  first  appeared  in  Natur,  1909-10,  Heft  6,  p.  94;  and  the  second,  which 
contains  coloured  plates  of  the  animals,  in  the  lecture  already  referrred  to,  12 
Flugschr.  d.  Deut.  Ges.f.  Zilchtungkunde,  Berlin,  1910,  p.  26.  In  the  paper  in  Mendel 
Festschrift,  1911,  the  subject  is  continued,  but  no  more  is  added  as  to  this  part  of 
the  experiment. 


eo8  PROBLEMS  OF  GENETICS 

one  in  which  the  yellow  of  the  body  is  irregularly  distributed  in 
spots  and  one  in  which  this  yellow  is  arranged  for  the  most  part 
in  two  longitudinal  bands  which  may  be  continuous  or  inter- 
rupted. The  spotted  form  is,  as  he  showed  me,  an  eastern  variety, 
and  the  striped  form  belongs  to  western  Europe.  Mr.  E.  G. 
Boulenger22  has  since  published  a  careful  account  of  the  distri- 
bution of  the  two  forms.  The  spotted  he  regards  as  the  typical 
form,  var.  typica,  and  for  the  striped  he  uses  the  name  var. 
taeniata.  The  typical  form  occupies  eastern  Europe  in  general, 
including  Austria  and  Italy,  extending  as  far  west  as  parts  of 
eastern  France.  The  var.  taeniata  is  found  all  over  France, 
excepting  parts  of  the  eastern  border,  Belgium  and  western 
Germany,  Spain  and  Portugal.  Of  the  very  large  series  examined 
there  was  only  one  specimen  (Lausanne)  which  could  not  with 
confidence  be  referred  to  one  or  other  of  the  two  varieties. 
Mr.  E.  G.  Boulenger  points  out  that  both  varieties  inhabit  very 
large  areas,  and  live  on  soils  of  most  different  colours  and  com- 
positions. Both  are  liable  to  variations  in  the  amount  and  the 
shade  of  the  yellow,  but  that  any  suggestion  that  taeniata  belongs 
especially  to  yellow  soils  and  typica  to  black  soils  is  altogether 
inadmissible.  He  expresses  surprise  that  Kammerer  should  not 
allude  to  these  peculiarities  in  the  geographical  distribution  of 
the  two  forms.  He  suggests  further  that  it  is  more  likely  that 
some  mistake  occurred  in  Kammerer's  observations  than  that  the 
east  European  typica  should,  in  the  course  of  a  generation,  have 
been  transformed  into  the  west  European  taeniata  by  the  influ- 
ence of  yellow  clay  soil. 

In  his  last  paper  on  the  subject  Kammerer  states  incidentally23 
that  he  has  found  the  striped  form  recessive  to  the  spotted.  No 
evidence  for  this  statement  is  given,  and  I  have  not  found  any 
other  reference  to  crosses  effected  between  the  two  natural  types. 
If,  however,  this  representation  is  correct,  it  is  conceivable  that 
the  production  of  taeniata  from  typica  was  in  fact  the  re-ap- 
pearance of  a  recessive  form.  The  plate  which  Kammerer  gives 
in  illustration  of  his  modified  parent  figures  a  single  animal  at 
four  stages,  and  though  it  is  certainly  more  like  the  spotted  than 

ME.  G.  Boulenger,  Proc.  Zool.  Soc.t  191 1,  p.  323- 
28  Mendel  Festschrift,  1911,  p.  84. 


ADAPTATION  209 

the  striped  form,  it  has  a  certain  suggestion  of  the  striped  arrange- 
ment, such  as  I  can  well  imagine  being  produced  in  the  hetero- 
zygote.24 

In  continuation25  of  the  experiments  on  the  colour  of  S.  macu- 
losa  Kammerer  publishes  an  account  of  elaborate  experiments 
in  grafting  ovaries  of  the  various  forms,  modified  and  unmodified, 
into  each  other,  and  describes  the  offspring  which  followed. 
Before  pursuing  this  part  of  the  inquiry  I  am  disposed  to  wait 
until  the  earlier  steps  have  been  made  much  more  secure  than 
they  yet  are. 

More  recently  Kammerer  has  published  similar  statements  in 
regard  to  the  inheritance  of  characters  induced  in  various  lizards 
by  keeping  them  in  abnormal  temperatures,  high  and  low.  The 
changes  induced  affected  in  some  species  the  colours,  in  others 
the  reproductive  habits.  Respecting  these  examples  I  feel  the 
same  scepticism  that  I  have  indicated  in  regard  to  the  others, 
somewhat  heightened  by  the  fact  that  insufficient  evidence  is 
given  both  regarding  the  behaviour  of  these  various  species  in 
captivity  when  not  subjected  to  abnormal  temperatures,  and 
in  the  wild  state. 

Respecting  this  part  of  the  evidence  Mr.  G.  A.  Boulenger  has 
lately  published  a  criticism26  from  which  I  extract  the  following 
passages.  Referring  to  a  previous  note27  on  the  question  of  the 
melanism  of  the  various  insular  forms  of  Lacerta  muralis  he 
writes:  "  I  also  alluded  (/.  c.)  to  the  theories  that  have  been 
propounded  to  explain  the  melanism  of  various  insular  forms. 
This  is  a  subject  which  has  been  lately  taken  up  by  Dr.  Kam- 
merer at  the  Biologische  Versuchsanstalt  in  Vienna,  and  he  claims 
to  have  produced  nigrinos  artificially  by  a  very  strong  elevation 
of  the  temperature,  accompanied  by  extreme  dryness.  Dr. 
Werner28  has  already  opposed  his  own  experiments  to  those  of 
Kammerer,  artificial  melanism  having  been  produced  by  him  in 
Lacerta  oxycephala  by  keeping  two  very  light  specimens  from 

24 12  Flugschrift.  Detit.  Ges.  Ziichtungskunde,  1910,  Fig.  15,  P.  Reihe. 

25  Mendel  Festschrift,  1911,  p.  83. 

26  Field,  1912,  30  March. 

27  Ibid.,  1904,  p.  863. 

28  Mitth.  Naturw.  Ver.  a.  d.  Univ.  Wien,  1908,  p.  53. 

IS 


2io  PROBLEMS  OF  GENETICS 

Ragusa  for  a  whole  summer  in  very  damp  conditions.  Neither  is 
Kammerer's  theory  in  accordance  with  the  distribution  of  the 
black  lizards,  as  pointed  out  by  Werner.  Kammerer  also  finds 
that  those  forms  which  are  known  to  produce  melanic  races  in  a 
state  of  nature,  lend  themselves  more  readily  than  the  others 
to  the  success  of  his  experiments.  But  he  shows  himself  misin- 
formed when  he  states  that  the  variety  called  Lacerta  fiumana 
belongs  to  the  category  of  those  of  which  black  forms  are  not 
known.  He  overlooks  the  fact,  first  pointed  out  by  Scherer  in 
1904,  and  which  I  can  confirm,  that  the  black  lizard  from  Meli- 
sello  near  Lissa  in  the  Adriatic  is  unquestionably  derived  from 
the  lizard  from  Lissa,  which  he  correctly  regards  as  not  separable 
from  L.  fiumana.  .  .  ." 

"  Another  colour  modification  which  Dr.  Kammerer  states 
that  he  obtained  by  raising  the  temperature  is  the  assumption  by 
the  female  of  the  typical  Lacerta  muralis  of  the  bright  red  colour 
of  the  lower  parts  which  often  distinguishes  the  male  from  the 
female,  and  which  was  not  shown  by  the  individuals  of  the  latter 
sex  kept  by  him  under  normal  conditions.  He  quotes  various 
authorities  to  show  that  the  lower  parts  are  never  red  in  the 
females,  but  he  has  omitted  to  consult  others  who  say  the  con- 
trary. Thus  Bedriaga  (1878  and  1879)  remarks  that  a  so-called 
var.  rubriventris  of  the  typical  wall  lizard  has  the  lower  parts  red 
in  both  sexes."29 

In  reading  such  papers  as  those  of  Semon  or  Kammerer  the 
thought  uppermost  in  my  mind  is  that  to.  multiply  illustrations 
of  supposed  transmission  of  acquired  characters  is  of  little  use 
until  some  one  example  has  been  thoroughly  investigated.  If 
we  had  certain  assurance  that  even  a  single  unimpeachable  case 
could  be  repeated  at  will,  the  whole  matter  would  assume  a  more 
serious  aspect.  If,  for  instance,  Kammerer  were  able  to  show  us 
Alytes  males  with  horny  pads  on  their  hands,  it  would  be  some- 
thing tangible;  still  more,  if  the  experiment  were  repeated  by 
others  until  no  doubt  remained  that  the  offspring  of  Alytes  which 
had  bred  in  water  for  some  three  generations  did  acquire  these 

29  As  to  the  variations  of  Lacerta  muralis  in  Western  Europe  and  North  Africa 
see  Boulenger,  G.  A.,  Trans.  Zool.  Soc.,  1905,  vol.  XVII,  p.  351. 


ADAPTATION  211 

pads  and  that  they  could  transmit  these  novelties  to  descendants 
raised  in  normal  conditions.  Till  evidence  of  this  kind  is  pub- 
lished by  at  least  two  independent  observers  investigating  similar 
material,  I  find  it  easier  to  believe  that  mistakes  of  observation  or 
of  interpretation  have  been  made  than  that  any  genuine  trans- 
mission of  acquired  characters  has  been  witnessed. 

Meanwhile  there  is  no  denying  that  the  origin  of  adaptational 
features  is  a  very  grave  difficulty.  With  the  lapse  of  time  since 
evolutionary  conceptions  have  become  a  universal  subject  of  study 
that  difficulty  has,  so  far  as  I  see,  been  in  nowise  diminished. 
But  I  find  nothing  in  the  evidence  recently  put  forward  which 
justifies  departure  from  the  agnostic  position  which  most  of  us 
have  felt  obliged  to  assume.30 

APPENDIX  TO  CHAPTER  IX. 

Professor  G.  Klebs,  as  is  well  known  to  students  of  evolution- 
ary phenomena,  has  for  several  years  been  engaged  in  investi- 
gations relating  to  the  inheritance  of  acquired  characters.  In 
his  many  publications  on  the  subject  the  issue  has  always  been 
represented  as  more  or  less  uncertain. 

Desiring  to  know  how  the  matter  now  stands  according  to 
Professor  Klebs'  present  judgment  I  wrote  to  him  asking  him  to 
favour  me  with  a  brief  general  statement.  This  he  most  kindly 
sent  in  a  letter  dated  8th  July,  1912. 

As  such  a  statement  will  be  read  with  the  greatest  interest 
by  all  who  are  watching  the  progress  of  these  studies  I  obtained 
permission  to  publish  it  as  follows: 

8.  Juli  1912 

Ihre  liebenswurdige  Anfrage  will  ich  sehr  gern  beantworten, 
obwohl  ich  sie  nicht  so  beantworten  kann  wie  ich  erwiinschte. 
Ihr  Skepticismus  in  der  Frage  der  Uebertragung  erworbener 
Charactere  auf  die  Nachkommen  ist  nur  zu  berechtigt.  Meine 
Versuche  mit  Veronica  sind  nicht  beweisend,  da  es  mir  bisher 
nicht  gelungen  ist  eine  einigermasse  konstante  Varietat  mit 
verlaubten  Inflorescenze  zu  erzeugen.  In  Bezug  auf  mein 
Semper  vivum  bin  ich  allerdings  noch  heute  der  Meinung  dass 

80 As  to  the  experiments  of  Klebs  relating  to  the  transmission  of  acquired  char- 
acters, see  Appendix. 


212  PROBLEMS  OF  GENETICS 

die  starke  kunstliche  Veranderung  der  Bliite  einen  Einfluss  auf 
einzelnen  Nachkommen  gehabt  hat.  Ich  habe  seither  nichts 
dariiber  veroff entlicht :  die  Mehrzahl  der  anormalen  gefiillten 
Bliiten  war  leider  steril.  Von  einem  weniger  veranderten  Ex- 
emplar erhielt  ich  einige  Samlinge,  aber  sie  haben  noch  nicht 
gebliiht.  Es  kann  sich  in  diesem  Falle  nur  um  eine  Nachwirkung 
in  der  ersten  Generation  handeln,  vergleichbar  jenen  Fallen  in 
denen  Samen  von  Baumen  aus  den  hohen  Alpen  in  der  Ebene 
gewisse  Nachwirkungen  zeigen.  Aber  es  ist  bisher  kein  sicherer 
Fall  bekannt  in  den  der  kunstliche  herbeigefiihrte  Charakter 
mehrere  Generationen  hindurch  unter  der  gewohnlichen  "normalen" 
Bedingungen  iibertragen  worden  ist. 

Auf  der  andere  Seite  sind  diese  negativen  Resultaten  nicht 
entscheidend.  Denn  wie  wenig  ist  in  dieser  Beziehung  iiber- 
haupt  ernstlich  versucht  worden!  Und  zweifellos  geht  die 
Sache  nicht  so  einfach. 

Ich  versuche  es  mit  anderen  Pflanzen  weil  ich  der  Meinung 
bin  dass  es  moglich  sein  musse  wenigstens  solche  neuen  Varie- 
taten  zu  erzeugen,  wie  sie  die  Gartenvarietaten  entsprechen. 

Aber  bis  jetzt  leider  sind  die  Versuche  nicht  gelungen,  weder 
mir  noch  irgend  einem  anderen. 


CHAPTER  X 

EFFECTS  OF  CHANGED  CONDITIONS  CONTINUED 
THE  CAUSES  OF  GENETIC  VARIATION 

IN  the  last  chapter  we  examined  some  of  the  evidence  offered 
in  support  of  the  belief  that  adaptation  in  highly  organised  forms 
is  a  consequence  of  the  inheritance  of  adaptative  changes  induced 
by  the  influence  of  external  conditions.  The  state  of  knowledge 
of  this  whole  subject  is,  as  I  have  said,  most  unsatisfactory, 
chiefly  for  the  reason  that  in  none  of  the  cases  which  are  alleged 
to  show  a  positive  result  have  two  observers  been  over  the 
same  ground,  or  as  yet  confirmed  each  other.  In  the  wider 
consideration  respecting  the  causes  of  variation  at  large  we  find 
ourselves  still  in  the  same  difficulty.  The  study  has  thus  far 
proved  sadly  unfruitful.  In  spite  of  the  considerable  efforts 
lately  made  by  many  observers  to  induce  genetic  variation  in 
highly  organised  plants  or  animals,  and  though  successes  have 
occasionally  been  announced,  I  do  not  know  a  single  case  which 
has  been  established  and  confirmed  in  such  a  way  that  we  could 
with  confidence  expect  to  witness  the  alleged  phenomena  if  we 
were  to  repeat  the  experiment.  Abundant  illustrations  are 
available  in  which  individuals  exposed  to  novel  conditions  mani- 
fest considerable  changes  in  characters  or  properties,  but  as  yet 
there  is  no  certain  means  of  determining  that  germ-cells  of  a  new 
type  shall  be  formed. 

Of  the  direct  effect  of  conditions  the  lower  organisms,  espe- 
cially bacteria,  offer  the  best  examples,  the  alterations  of  virulence 
which  can  be  produced  in  so  many  distinct  ways  being  the  most 
striking  and  familiar.  That  attenuation  of  virulence  can  be 
produced  by  high  temperatures  or  by  exposure  to  chemical 
agents,  and  that  this  diminution  in  virulence  may  remain  perma- 
nent is,  from  our  point  of  view,  not  surprising ;  but  the  fact  that 
in  many  cases  the  full  virulence  can  by  suitable  cultivation  be 

213 


214  PROBLEMS  OF  GENETICS 

restored  is  difficult  to  understand.  Similar  variations  have  been 
observed  in  power  of  pigment  production  and  other  properties. 

These  phenomena  naturally  raise  the  question  whether  any 
cases  of  apparent  loss  of  factors  in  higher  forms  may  be  com- 
parable. 

The  subject  of  variations  in  the  lower  organisms  and  their 
dependence  on  conditions  is  a  highly  special  one,  and  I  have  no 
knowledge  which  can  justify  me  in  offering  any  discussion  of 
them,  but  I  understand  that  hitherto  little  beyond  empirical 
recognition  of  the  phenomena  has  been  attempted.  A  useful 
summary  of  observations  made  by  many  investigators  was  lately 
published  by  Hans  Pringsheim,1  who  enumerates  the  different 
agencies  which  have  been  observed  to  produce  modifications, 
and  the  various  ways  in  which  these  changes  are  manifested. 
One  of  the  most  comprehensive  studies  of  the  subject  from  the 
genetic  point  of  view  is  that  made  by  F.  Wolf.2  In  his  extensive 
cultivations  of  Bacillus  prodigiosus,  Staphylococcus  pyo genes  and 
Myxococcus  he  succeeded  in  producing  many  strains  with  modi- 
fied properties.  In  most  of  these  the  modifications  arose  in 
consequence  of  the  application  of  high  or  low  temperatures  or  of 
the  addition  of  various  chemical  substances  to  the  culture-media. 
Some  of  the  variations,  which  are  for  the  most  part  in  the  powers 
of  pigment-formation,  persisted  when  the  strains  were  returned 
to  normal  conditions,  and  others  did  not.  In  reference  especially 
to  the  variations  witnessed  in  the  Cocci  the  reader  should  consult 
the  critical  account  of  variation  in  that  group  published  by  the 
Winslows,3  where  much  information  on  the  subject  is  to  be 
found.  The  authors  attempted  to  determine  the  systematic 
relationships  of  the  several  forms,  as  far  as  possible,  by  the 
application  of  statistical  methods.  The  result  is  interesting  as 
showing  that  the  problem  of  species  in  its  main  features  is  pre- 
sented by  these  organisms  in  a  form  identical  with  that  which 
we  know  so  well  in  the  higher  animals  and  plants,  whatever 

1  Pringsheim,  H.,  Die  Variabilitdt  niederer  Organismen,  Berlin,  1910. 

2  F.  Wolf,  Modifikationen  u.  Mutationen  von  Bakterien,  Zts.  F.  indukt.  Abstam. 
u.  Vererbungslehre,  II,  1909,  p.  90. 

3  Winslow,   C.  E.  A.  and  A.  R.t  Systematic  Relationships  of    the  Coccaceae. 
New  York.     1909. 


CAUSES  OF  GENETIC  VARIATION  215 

properties  be  selected  as  the  diagnostic  characters.  There  are 
many  types  perfectly  distinct  and  others  which  intergrade. 
Some  of  the  types  change  greatly  with  conditions  while  others  do 
not.  This  is  exactly  what  we  encounter  whenever  we  study  the 
problem  of  species  on  an  extended  scale  among  the  higher  forms 
of  life. 

There  is  now  practically  complete  agreement  among  bac- 
teriologists that  the  observations  made  first  by  Massini  on  the 
change  in  color  of  Bacterium  coli  mutabile  grown  in  Endo's 
medium,  associated  with  the  acquisition  of  the  power  to  ferment 
lactose,  are  perfectly  reliable  and  free  from  possibilities  of  mis- 
take. The  work  has  been  extended  and  confirmed  by  many 
workers,  especially  R.  Miiller,  who  finds  that  this  bacterium  can 
similarly  acquire  and  maintain  the  power  to  ferment  other 
sugars.  A  careful  account  of  the  whole  subject  written  by  Miiller 
for  the  information  of  biologists  will  be  found  in  Zts.fur  Abstam- 
mungsl.,  VIII,  1912.  After  discussing  the  biological  significance 
of  the  facts,  he  concludes  with  a  caution  to  the  effect  that  bacteria 
are  so  different  from  all  other  living  things  that  generalizations 
from  their  behavior  must  not  be  indiscriminately  applied  to 
animals  and  plants. 

In  all  work  with  this  class  of  material  there  is  obviously 
danger  of  error  through  foreign  infection  of  the  cultures,  but 
there  can  be  no  doubt  that  though  some  of  the  "mutations" 
recorded  may  be  due  to  this  cause,  the  majority  of  the  instances 
observed  under  stringent  conditions  are  genuine. 

Another  and  equally  serious  difficulty  besetting  work  with 
bacteria  and  fungi  cultivated  from  spores  is  that  the  appearance 
of  variation  may  in  reality  be  due  to  the  selection  of  a  special 
strain  previously  living  masked  among  other  strains.  This 
possibility  must  be  remembered  especially  in  those  instances 
wrhich  are  claimed  as  exemplifying  the  effects  of  acclimatisation. 
Manifestly  this  consideration  can  be  urged  with  most  force  when 
the  strain  which  gave  rise  to  the  novelty  was  not  raised  from  a 
single  individual  spore.  Moreover,  when  once  the  possibility  of 
spontaneous  variation  is  admitted,  it  must  be  difficult  to  be  quite 
confident  that  any  given  variation  observed  is  in  reality  due  to 


216  PROBLEMS  OF  GENETICS 

the  novel  conditions  applied,  and  as  I  understand  the  evidence, 
the  appearance  of  the  mutational  forms  does  not  with  any 
regularity  follow  upon  the  application  of  the  changed  conditions. 

Researches  into  the  variation  of  these  lower  forms  will,  no 
doubt,  be  continued  on  a  comprehensive  scale.  So  long  as  the 
instances  recorded  are  each  isolated  examples  it  is  impossible  to 
know  what  value  they  possess.  If  they  could  be  coordinated  in 
such  a  way  as  to  provide  some  general  conception  of  the  types  of 
variation  in  properties  to  which  bacteria,  or  any  considerable 
group  of  them,  are  habitually  liable,  the  knowledge  might 
greatly  advance  the  elucidation  of  genetic  problems. 

Of  mutational  changes  directly  produced  with  regularity  in 
micro-organisms  by  treatment,  the  experiments  with  trypano- 
somes  provide  some  of  the  clearest  examples.  A  summary  of 
the  evidence  was  lately  published  by  Dobell,4  from  which  the 
present  account  is  taken.  The  most  definite  fact  of  this  kind 
established  is  that  certain  dyes  introduced  into  the  blood  of  the 
host  have  the  effect  of  destroying  the  small  organ  known  as  the 
"  kineto-nucleus  "  in  the  trypanosomes.  The  trypanosomes  thus 
altered  continue  to  breed,  and  give  rise  to  races  destitute  of 
kinetonuclei.  This  observation  was  originally  made  by  Wer- 
bitzki  and  has  been  confirmed  by  several  observers.  The  exact 
way  in  which  this  alteration  is  effected  in  the  trypanosomes  is  not 
quite  definitely  made  out,  but  there  is  good  reason  for  supposing 
that  the  dyes  have  a  direct  and  specific  action  upon  the  kineto- 
nucleus  itself,  and  circumstances  make  it  improbable  that  in 
some  division  a  daughter-organism  without  that  body  is  pro- 
duced, or  that  any  selection  of  a  pre-existing  defective  variety 
occurs. 

Ehrlich  has  suggested  with  great  probability  that  the  dyes 
which  possess  this  action  owe  it  to  the  fact  that  they  have  the 
particular  chemical  linkage  which  he  calls  "ortho-quinoid."  In 
outward  respects,  such  as  motility  and  general  appearance,  the 
modified  organisms  are  unchanged,  but  their  virulence  is  di- 
minished. As  regards  the  possibility  of  the  defective  strain  re- 

4C.  C.  Dobell,  Jour.  Genetics,  1912,  II,  p.  201,  where  full  references  are  given. 
Still  more  recently  the  same  author  has  contributed  an  excellent  summary  of 
the  evidence  relating  to  bacteria  (ibid.,  II.  1913,  p.  325). 


CAUSES  OF  GENETIC  VARIATION  217 

acquiring  the  kinetonucleus,  Werbitzki  states  that  in  one  case 
passage  through  50  animals  and  treatment  with  dyes  left  the 
strain  unaltered;  but  that  in  another  case  at  the  sixteenth 
passage  7  per  cent,  of  the  trypanosomes  were  found  to  have 
re-acquired  the  organ,  and  in  subsequent  passages  the  percentage 
increased,  until  at  the  twenty-seventh  passage  practically  all  had 
re-acquired  it.  Kudicke,  however,  in  similar  experiments  did  not 
succeed  in  causing  re-acquisition  by  transplantation. 

By  the  action  of  various  drugs  and  antibodies  races  of  try- 
panosomes resistant  to  those  substances  have  been  obtained. 
These  breed  true,  at  least  when  kept  in  the  same  species  of  animal 
in  which  the  resistance  was  acquired.  As  to  whether  change  of 
virulence  is  produced  by  passage  through  certain  animals  or  not, 
there  is  as  yet  no  general  agreement. 

Other  changes,  especially  in  size  and  some  points  of  structure, 
are  said  to  occur  when  certain  trypanosomes  proper  to  mammals 
are  passed  through  cold-blooded  vertebrates  (Wendelstadt  and 
Fellmer),  and  it  is  stated  that  these  changes  persist,  but  the 
observations  have  not  yet  been  confirmed. 

Experiments  lately  conducted  by  Woltereck  with  Daphnia 
are  interesting  as  having  given  a  definite  positive  result,  in  so  far, 
at  least,  as  the  ova  were  affected  by  conditions  before  leaving  the 
bodies  of  the  parent  individuals.  The  observations  relate  to 
the  offspring  resulting  from  partheno genetic  eggs.  Females 
bearing  ephippia  (fertilised  eggs)  were  isolated  until  the  ephippia 
were  dropped,  and  in  this  way  the  offspring  of  fertilisation  were 
excluded.  Males,  of  course,  appeared  from  time  to  time  in  the 
cultures,  but  as  fertilised  eggs  were  rejected,  their  presence  did 
not  disturb  the  result.  The  most  remarkable  observations 
related  to  Daphnia  longispina. 

This  species  as  found  in  the  lower  lake  at  Lunz  had  the  front 
end  of  the  body  blunt  and  nearly  round  in  profile;  but  on  being 
cultivated  in  a  warm  temperature  and  with  abundant  nourish- 
ment the  front  end  of  the  body  became  produced  into  an  elon- 
gated "helmet,"  as  Woltereck  calls  it.  Experiment  showed  that 
the  change  was  primarily  due  to  the  abundance  of  food,  and  owing 
to  temperature  in  a  subordinate  degree. 


'2i8  PROBLEMS  OF  GENETICS 

This  distinction  arose  as  soon  as  the  species  was  taken  into 
the  hothouse,  but  when  the  modified  individuals  were  put  back 
into  the  original  conditions,  a  lower  temperature  and  scanty 
food-supply,  the  next  generation  returned  to  their  original  form. 
After  being  cultivated  for  two  years  and  about  40  generations  in 
the  more  favourable  conditions,  when  similarly  put  back  into 
the  lower  temperature  with  scanty  food  the  first  generation  born 
in  these  conditions  was  helmeted  like  the  modified  parents. 
Woltereck  is  of  opinion  that  the  ova  were  still  unformed  at  the 
time  the  parents  were  put  back,  and  the  influence  of  the  favour- 
able conditions  upon  the  unformed  ova  he  speaks  of  as  a  "prae- 
induction."  The  effect  never  extended  beyond  the  one  gener- 
ation, after  which  the  strain  returned  to  its  original  state. 

The  fact  that  the  influence  on  the  offspring  was  not  mani- 
fested at  first  led  Woltereck  to  expect  that  by  more  prolonged 
cultivation  in  the  favourable  conditions  a  further  extension  of 
this  influence  would  be  produced,  but  this  expectation  was  never 
fulfilled,  though  the  attempt  was  made  again  and  again. 

Similar  experiments  were  made  with  Hyalodaphnia  cucullata, 
which  is  far  more  sensitive  to  cultural  influences,  and  in  nature 
manifests  a  considerable  elongation  of  the  helmet  as  a  seasonal 
modification,  but  the  results  were  essentially  the  same  as  in  the 
preceding  case,  no  modification  extending  beyond  the  first 
generation  born  after  the  restoration  to  normal  conditions? 

The  only  criticism  of  these  extremely  interesting  results  which 
suggests  itself  is  that  perhaps  the  original  appearance  of  the 
modification  was  not  in  reality  due  to  an  accumulated  effect  of 
the  conditions,  but  to  some  change  in  the  conditions  themselves 
which  was  not  noticed.  It  is  difficult  to  see  how  length  of 
time  or  even  the  lapse  of  several  generations  could  have  so  specific 
an  effect  on  the  race.  It  is  no  doubt  often  vaguely  supposed 
by  many  that  a  long  period  of  time  may  be  necessary  for  the 
effect  of  climate  or  of  other  environmental  conditions  to  be 
produced  in  an  organism  which  does  not  thus  respond  at  first. 
I  have  never  been  able  to  see  any  reason  for  this  opinion  nor  how 

5  See  Woltereck,  Verh.  d.  Deut.  ZooL  Ges.,  1909,  p.  no;  and  1911,  p.  142.  This 
is  a  subject  which  can  only  be  properly  appreciated  on  reference  to  the  original 
papers.  Several  complications  are  involved  to  which  I  have  not  here  alluded. 


CAUSES  OF  GENETIC  VARIATION  219 

it  is  to  be  translated  into  terms  of  physiological  fact,  and  I 
imagine  that  in  those  cases  in  which  the  lapse  of  time  is  really 
required  for  the  production  of  an  effect,  the  influence  of  the 
prolongation  is  rather  on  the  conditions  than  on  the  organisms. 
The  response  of  the  organisms  thus  probably  indicates  not  that 
the  creature  is  at  length  feeling  the  effects  because  of  their 
accumulated  action  on  itself,  but  that  the  conditions  have  at 
length  ripened. 

As  this  sheet  is  passing  through  the  press  Agar  has  published6 
an  abstract  of  evidence  as  to  another  comparable  case  in  a  par- 
thenogenetic  strain  in  the  daphnid,  Simocephalus  vetulus.  When 
fed  on  certain  abnormal  foods  the  shape  of  the  body  is  changed, 
the  edges  of  the  carapace  being  rolled  backwards  so  as  to  expose 
the  appendages.  The  offspring  of  animals  thus  modified  showed 
similar  modification  in  the  first,  and  to  a  very  slight  degree,  in 
the  second  generation,  though  the  original  mothers  were  removed 
to  normal  conditions  before  their  eggs  were  laid.  In  the  third 
generation  there  was  "a  very  pronounced  reaction  in  the  opposite 
direction."  Agar  suggests  that  the  change  may  be  due  to  some 
toxin-like  substances,  carried  on  passively  by  the  egg  into  the 
next  generation,  against  which  the  protoplasm  eventually  pro- 
duces an  anti-body. 

The  experiments  which  have  been  in  recent  years  regarded 
by  evolutionary  writers  as  the  most  conclusive  proof  that  direct 
environmental  action  may  produce  germinal  variation  are  those 
of  Professor  W.  L.  Tower,  of  Chicago,  on  Leptinotarsa,  the 
potato  beetles.  This  work  has  attained  considerable  celebrity 
and  has  been  generally  accepted  as  making  a  definite  extension 
of  knowledge.  After  frequently  reading  Tower's  papers  and 
after  having  been  privileged  to  see  some  of  the  experiments  in 
progress  (in  1907)  I  am  still  in  doubt  as  to  the  weight  which 
should  be  assigned  to  this  contribution. 

The  work  is  described  in  two  chief  publications,  the  first  of 
which  appeared  in  I9o6.7  This  treatise  contains  a  vast  amount 
of  information  about  numerous  species  and  varieties  of  these 

6Proc.  Roy.  Soc.,  B,  Vol.  86,  1913,  p.  113. 

7  An  Investigation  of  Evolution  in  Chrysomelid  Beetles  of  the  Genus  Leptinotarsa, 
Carnegie  Publications,  1906,  No.  48. 


220  PROBLEMS  OF  GENETICS 

beetles  which  the  author  has  observed  and  bred  in  many  parts  of 
their  distribution  throughout  the  United  States,  Mexico  and 
Central  America.  The  part  of  the  book  which  has  naturally 
excited  the  greatest  interest  is  that  in  which  Tower  states  that 
by  subjecting  the  beetles  to  change  in  temperature  and  moisture, 
he  caused  them  to  produce  offspring  quite  unlike  themselves, 
which  in  several  cases  bred  true. 

It  is  much  to  be  regretted  that  the  author  did  not  happen  to 
become  acquainted  with  Mendelian  analysis  at  an  earlier  stage 
in  the  investigation.  The  evidence  might  then  have  been 
handled  in  a  much  more  orderly  and  comprehensive  way,  and  a 
watch  would  have  been  kept  for  several  possibilities  of  error. 

The  headquarters  of  the  genus  is  evidently  as  Tower  states, 
in  Mexico  and  the  adjoining  countries.  In  this  region  there  is 
a  great  profusion  of  forms,  some  very  local,  some  as  for  instance 
the  well-known  decemlineata*  more  widely  spread.  The  dis- 
tinctions are  almost  all  found  in  peculiarities  of  colour  and 
pattern,  and  the  limits  of  species  are  even  more  indefinable  than 
is  usual  in  multiform  animals.  Tower  arranges  the  various  types 
into  seven  groups  of  which  the  one  most  studied  is  that  which 
he  calls  the  lineata  group.  To  this  group  belong  all  the  forms 
to  which  reference  is  here  made,  and,  as  I  understand,  they  differ 
among  themselves  entirely  in  size,  colour  and  pattern.  There 
is  no  suggestion  of  infertility  in  the  crosses  made  between  the 
several  forms  of  the  lineata  group ;  in  fact  they  present,  like  many 
Chrysomelidae,  a  good  example  of  what  most  of  us  would  now 
call  a  polymorphic  species,  consisting  of  many  types,  some  found 
existing  in  the  same  locality,  others  being  geographically  isolated. 

A  series  of  experiments  was  devoted  to  the  attempt  to  fix 
strains  corresponding  to  the  extremes  of  continuous  variations. 
For  example,  those  with  most  black  pigment  and  those  with 
least  black  taken  from  a  population  continuously  varying  in  this 
respect,  were  separately  bred;  but  almost  always  the  selection 
led  to  no  sensible  change  in  the  position  of  the  mean  of  the  popu- 

8  This  is  the  famous  Colorado  beetle  or  potato-bug,  which  has  caused  such  serious 
destruction  in  potato  crops.  There  seems  to  be  no  doubt  that  this  insect,  formerly 
unknown  in  the  eastern  States,  made  its  way  east  along  the  mining  trails  when  the 
west  was  opened  up. 


CAUSES  OF  GENETIC  VARIATION  221 

lation.  The  variations  in  these  cases  were  evidently  fluctu- 
ational.  In  some  instances,  however,  real  genetic  differences 
were  met  with,  and  strains  exhibiting  them  were,  as  usual, 
rapidly  fixed. 

Tower  points  out  that  several  of  the  varieties  (or  species,  as 
he  prefers  to  call  them)  were  obviously  recessive  to  decemlineata. 
This  is  most  clearly  demonstrated  in  the  case  of  the  form  called 
pallida,  which  is  a  pale  depauperated-looking  creature,  with  the 
orange  of  the  thorax  almost  white  and  the  eyes  devoid  of  pig- 
ment.9 This  form  behaved  as  an  ordinary  Mendelian  recessive, 
breeding  true  whenever  it  appeared  in  the  cultures,  or  when 
individuals  found  wild  were  studied  in  captivity.  A  black  form 
which  Tower  names  melanicum  was  similarly  shown  to  be  a 
Mendelian  recessive.  Wild  specimens  of  this  variety  of  opposite 
sexes  were  not  found  simultaneously  in  nature,  and  there  was  thus 
no  opportunity  of  breeding  them  together,  but  the  hereditary 
behaviour  was  seen  in  the  F2  generation  from  a  melanicum  found 
coupled  with  decemlineata.  Experiments  also  occurred  giving 
indication  that  a  variety  with  the  stripes  anastomosing  in  pairs 
(tortuosa),  was  another  recessive,  and  that  a  variety — called 
"  rubri-vittata" — gave  an  intermediate  FI  with  subsequent 
segregation.  All  these  are  forms  of  decemlineata  Stal. 

Similar  observations  were  made  regarding  forms  recessive  to 
multitaeniata  Stal.  Of  these  two  were  thrown  by  multitaeniata 
itself,  namely  a  form  named  by  Stal  melanothorax,  and  regarded 
by  him  as  a  species,  and  one  which  Tower  names  rubicunda  n.  sp. 
The  facts  proving  the  recessive  behaviour  of  their  several  forms 
will  be  found  in  the  following  places  in  Tower's  book: 

pallida,  pp.  273-278. 

melanicum,  p.  279. 

tortuosa,  p.  280. 

rubrivittata,  pp.  280-281. 

melanothorax  and  rubicunda,  pp.  283-285. 

Following  this  evidence  of  recessive  nature  of  the  six  forms 

9  This  is  indicated  in  the  coloured  plate,  but  I  have  not  found  any  explicit  state- 
ment to  this  effect  in  the  text,  and  am  not  sure  if  the  absence  of  pigment  was  re- 
garded as  complete. 


222  PROBLEMS  OF  GENETICS 

enumerated,  Tower  describes  experiments  showing,  as  he  believes, 
that  some  of  them  may  be  caused  to  appear  by  applying  special 
treatment  to  the  parents  during  the  "growth  and  fertilisation" 
(p.  287)  of  the  eggs.  The  most  striking  example  is  that  in  which 
4  males  and  4  females  of  decemlineata  were  kept  very  hot  (average 
35°  C.)  and  dry,  and  at  low  atmospheric  pressure  (19-21  inches). 
The  eggs  laid  were  restored  to  natural  conditions.  These  gave 
506  larvae,  from  which  emerged  14  normal,  82  pallida  and  2 
"  immaculo-thorax  "  viz.,  without  pigment  on  the  pronotum. 
The  account  of  the  rest  of  the  experiment  is  somewhat  involved, 
but  I  understand  that  the  pallida,  of  which  two  only  survived, 
behaved  as  normal  recessives  when  bred  to  the  type:  also  that 
the  parents,  after  having  laid  the  eggs  whose  history  has  been 
given,  were  restored  to  normal  conditions  and  laid  319  eggs  which 
gave  6 1  normals. 

In  another  case  normal  parents  laid  409  eggs  in  the  hot  and 
dry  conditions,  and  on  restoration  to  normal  conditions,  the 
same  parents  laid  840  eggs.  Then  409  eggs  gave  64  adults  as 
follows : 

Males  Females 

decemlineata 12  8 

pallida 10  13 

immaculothorax 2  3 

albida 9  7 

13  3i 

The  840  eggs  laid  in  normal  conditions  gave  123  normal  decem- 
lineata. 

Similar  experiments  were  made  with  multitaeniata  and  gave 
comparable  results,  the  two  recessives  (melanothorax,  rubicunda) 
being  produced  in  large  numbers  when  the  parents  were  subjected 
to  heat,  but  in  this  case  the  atmosphere  was  kept  saturated  with 
moisture,  instead  of  dry,  as  in  the  previous  instance.  The  same 
parents  transferred  to  normal  conditions  gave  normals  only. 

Lastly  the  form  undecimlineata  was  exposed  "to  an  extreme 
stimulus  of  high  temperature,  10°  C.  above  the  average,"  and  a 
dry  atmosphere,  with  the  result  that  from  190  eggs  there  emerged 
II  beetles,  all  of  the  form  angustovittata  Jacoby,  which  subse- 
quently bred  true  to  that  type  (see  p.  295). 


CAUSES  OF  GENETIC  VARIATION  223 

In  the  results  of  these  experiments,  as  described,  there  is  one 
feature  which  I  regard  as  quite  unaccountable.  Tower  makes  no 
comment  upon  it.  Indeed,  from  the  general  tenour  of  the  paper, 
I  infer,  not  only  that  he  does  not  perceive  that  he  is  recounting 
anything  contrary  to  usual  experience,  but  rather  that  he  regards 
the  result  as  conforming  to  expectations  previously  formed. 
The  point  in  question  is  the  genetic  behaviour  of  the  dominant 
normals  produced  under  the  abnormal  conditions.  These 
normals  were  the  result  of  the  breeding  of  parents  declared  to  be 
at  the  same  time  giving  off  many  recessive  gametes.  Some  of 
these  normals  must  be  expected  therefore  to  be  heterozygous 
unless  some  selective  fertilisation  occurs.  Nevertheless  in  every 
case  they  and  their  offspring  are  reported  to  have  continually 
bred  true.  I  allude  especially  to  the  tables  given  on  pp.  288, 289, 
292,  and  293.  Tower  does  not  mention  any  misgiving  about 
this  result,  and  I  think  he  regards  himself  as  recounting  phe- 
nomena in  general  harmony  with  the  ideas  of  mutation  expressed 
by  De  Vries.  This  they  may  be;  but  to  anyone  familiar  with 
analytical  breeding  the  course  of  these  experiments  must  seem  so 
surprising  as  to  call  for  most  careful,  independent  confirmation. 

In  I9io10  Tower  published  an  account  of  further  experiments 
with  Leptinotarsa.  The  work  described  related  to  two  subjects. 
Crosses  were  made  between  three  forms,  undecimlineata  Stal, 
signaticollis  Stal  and  "diver so,"  named  by  Tower  as  a  new 
species.  The  distinctions  between  these  three  depend  partly  on 
characters  of  the  adults  and  partly  on  those  of  the  larvae.  The 
adults  of  undecimlineata  and  diver sa  have  the  elytra  striped,  but 
the  elytra  of  signaticollis  are  unstriped.  The  larvae  of  sig- 
naticollis and  of  diver  sa  are  yellow,  but  those  of  undecimlineata 
are  white.11  Moreover,  in  signaticollis  and  diversa  the  black  in- 
creases in  the  third  stage  of  the  larvae  to  form  transverse  bands 
which  are  absent  in  undecimlineata.  The  general  course  of  the 
experiments  shows  that  these  differences  may  be  approximately 

10  Biol.  Bull.,  XVIII,  1910,  p.  285. 

11  This  description  does  not  quite  agree  with  the  representation  of  the  larvae 
in  PI.  17  of  the  book  Evolution  in  the  Genus  Leptinotarsa  for  there  the  larva  of 
undecimlineata  is  shown  as  white  in  the  second  stage,  but  yellowish  in  the  third 
stage;  perhaps  there  is  an  error  in  printing. 


224  PROBLEMS  OF  GENETICS 

represented  as  due  to  the  action  of  three  factors,  any  of  which 
may  be  independently  present  or  absent.  The  stripings  of  the 
elytra  and  of  the  larvae  are  each  due  to  a  separate  factor.  As 
regards  the  distinction  between  the  yellow  and  the  white  larvae 
the  evidence  does  not  prove  that  there  is  decided  dominance  of 
either  colour  and  I  infer  that  the  heterozygotes  are  often  inter- 
mediate. 

The  chief  contribution  which  this  new  paper  claims  to  make 
relates  to  differences  in  the  results  which  ensue  from  crosses 
effected  between  these  three  types  at  different  average  tempera- 
tures. 

We  are  first  concerned  with  four  experiments  which  I  number 

(I),  (2),  (3),  (4):. 

1.  Signaticollis  9  X  diver  so,  o71  bred  at  an  average  tempera- 
ture of  80°  F.  by  day  and  75°  F.  by  night,  gave  two  groups  in 
about  equal  numbers.     The  first  (49)  was  pure  signaticollis  and 
bred  true.     The  second  (53)  was  of  an  intermediate  type,  which 
on  being  bred  together  gave  the  typical  Mendelian  result — I  sig.: 
2  intermediate:  I  div. 

2.  Next,  as  the  account  originally  stood  in  the  published 
paper,  we  are  told  that  sig  9   X  div  c?  bred  together  at  a  day- 
temp,  average  75°  F.  and  night  average  50°  F.  gave  an  inter- 
mediate only,  which  subsequently  produced  a  normal  1:2:1 
ratio.     The  two  crosses  were  repeated  eleven  times  with  identical 
results. 

In  a  further  experiment  (3)  signaticollis  9  X  diver sa  <?  were 
bred  under  the  same  conditions  as  those  used  in  expt.  (i).  They 
again  gave  sig.  and  intermediates  as  before  in  fairly  equal  numbers. 
The  sig.  as  before  bred  true,  and  the  intermediate  gave  I  :  2  :  I , 
all  exactly  as  in  expt.  (i). 

In  expt.  (4)  the  same  parents  used  in  (3)  were  again  mated 
under  conditions  of  expt.  (2)  at  the  lower  temperature,  and  this 
time  gave  signaticollis  exclusively,  which  bred  true  for  four 
generations.  This  experiment  was  repeated  seven  times  with 
uniform  results. 

Diagrams  are  given  representing  all  these  histories  in  graphic 
fashion. 


CAUSES  OF  GENETIC  VARIATION  225 

From  these  observations,  Tower  concludes  that  the  determi- 
nation of  dominance,  and  the  ensuing  type  of  behaviour,  is  clearly 
a  function  of  the  conditions  incident  upon  the  combining  germ 
plasms. 

It  will  be  observed  that  expts.  (i)  and  (3)  gave  identical 
results  but  (2)  and  (4),  though  much  the  same  conditions  were 
applied,  are  at  variance,  for  (2)  gave  all  intermediates,  while 
(4)  gave  all  signaticollis.  In  Amer.  Nat.,  XLIV,  1910,  p.  747, 
Professor  T.  D.  A.  Cockerell  commented  on  this  paper  of  Tower's 
and  pointed  out  that  there  must  be  an  error  somewhere,  for  when 
he  discusses  these  experiments  Tower  speaks  of  (2)  and  (4)  as 
confirming  each  other.  To  this  Tower  replied12  that  there  had 
been  a  mistake.  He  states  that  in  preparing  the  paper  "certain 
minor  experiments  were  taken  from  a  larger  series  and  combined 
to  illustrate  a  general  point  in  the  behaviour  of  alternative 
characters  in  inheritance,"  and  that  expt.  (2)  was  introduced 
inadvertently  in  place  of  another  which  he  desires  to  substitute. 
In  this,  which  I  number  (5),  signaticollis  9  X  diversa  cf  from 
exactly  the  same  stocks  as  those  used  in  (i),  were  mated  at  the 
lower  temperatures  specified  for  (2),  day  average  75°  F.,  night 
average  50°  F.  These  gave  all  of  the  signaticollis  type  with  a 
narrow  range  of  variability,  which  bred  true,  in  some  cases  to  F«. 
Tower  says  he  has  repeated  this  experiment  six  times  with  identi- 
cal results. 

Nevertheless  he  proceeds  to  say  that  the  description  of  expt. 
(2),  which  was  repeated  eleven  times  with  identical  results,  was 
correct  "as  far  as  given."  That  experiment  was  "  from  a  second 
series  of  cultures  parallel  to  the  one  given,  but  in  which  there  are 
other  factors  involved,  which  in  H.  410  [my  (2)]  are  productive 
of  a  typical  Mendelian  behaviour."  He  adds  he  does"  not  care 
at  this  time  to  make  any  statement  of  what  these  factors  are, 
nor  of  their  relations  to  the  behaviours  given  in  the  H.  409,  H.  41 1 , 
H.  409/11  series  [my  (i),  (5)  and  (3)-(4)]  which  are  the  simplest 
and  most  easily  presented  series  obtained  in  the  crossing  of 
signaticollis  and  diversa." 

Professor  Cockerell's  intervention  has  thus  elicited  the  fact 

«  Biol.  Bull.,  XX,  1910,  p.  67. 
16 


226  PROBLEMS  OF  GENETICS 

that  we  have  as  yet  only  a  small  selected  part  of  the  evidence 
before  us,  even  as  concerning  the  effect  of  temperature  on  the 
cross  between  signaticollis  9  X  diver sa  cf .  We  learn  that  at 
the  lower  temperatures  the  result  was  eleven  times  the  expected 
one,  and  six  times  an  unexpected  one ;  further,  that  we  owe  it  to 
the  author's  inadvertence  that  we  have  come  to  hear  of  the 
expected  result  at  all,  and  that  though  he  knows  the  factors 
which  determine  the  discrepancy,  he  declines  for  the  present  to 
name  them.  In  these  circumstances  we  can  scarcely  venture  as 
yet  to  estimate  the  significance  of  these  records. 

The  paper  goes  on  to  recount  somewhat  comparable,  but  more 
complex  instances  in  which  the  descent  of  the  colour  of  adults  and 
of  larvae  was  affected  by  temperature  in  crosses  between  un- 
decimlineata  and  signaticollis.  As  they  stand  the  results  are 
very  striking  and  unexpected,  but  I  think,  in  view  of  what  has 
been  admitted  respecting  the  former  part  of  the  paper,  full  dis- 
cussion may  be  postponed  till  confirmation  is  forthcoming. 

One  feature,  however,  calls  for  remark.  This  second  paper  is 
written  apparently  without  any  reference  to  the  discoveries 
related  by  Tower  in  his  previous  book,  to  which  no  allusion  is 
made.  This  is  most  noticeable  in  the  case  of  an  experiment  in 
which  (  p.  296,  H.  yooA  )  undecimlineata  9  (the  dominant) 
was  mated  to  signaticollis  o71  with  the  result  that  all  the  offspring 
were  undecimlineata  and  bred  true  to  that  type  (Parthenogenesis 
was  tested  for,  but  never  found  to  occur).  This  experiment  was 
made  at  a  temperature  averaging  95°  F.  =1=  3.5°  by  day  and  89°  F. 
=t=  4.8°  by  night,  and  in  a  humidity  given  as  84  per  cent,  by  day 
and  100  per  cent,  by  night;  but  in  the  previous  book  (p.  294)  we 
are  told  that  pure  undecimlineata  bred  together  "under  an  ex- 
treme stimulus  of  high  temperature,  10°  C.  above  the  average" 
and  a  relative  humidity  of  40  per  cent,  gave  n  beetles  only, 
all  angustovittata.  But  reference  to  the  Plate  16,  Fig.  2,  shows 
that  angustovittata  must  be  exceedingly  like  signaticollis,  having, 
like  it,  the  elytral  stripes  obsolete,  and  if  there  is  any  marked 
difference  at  all,  it  can  only  be  in  the  larvae.  It  seems  strange 
that  if  undecimlineata  really  gives  off  ova  of  this  recessive  type  at 
high  temperatures,  the  fact  should  not  be  alluded  to  in  connection 


CAUSES  OF  GENETIC  VARIATION  227 

with  expt.  H.  7OoA,  where,  as  the  father  was  signaticollis,  having 
the  same  recessive  character,  their  appearance  might  have  been 
expected  not  to  pass  unobserved.  The  temperature  in  the 
older  experiment  is,  of  course,  not  given  with  the  great  accuracy 
used  in  the  second ,  and  it  may  have  been  higher  still.  The  humid- 
ity also  was  widely  different.  Still,  in  discussing  the  phenomena 
we  should  expect  some  reference  to  the  very  remarkable  and 
closely  cognate  discovery  which  Tower  himself  had  previously 
reported  in  regard  to  the  same  species.13 

The  hesitation  which  I  had  come  to  feel  respecting  these  two 
publications  of  Tower's  has  been,  I  confess,  increased  by  the 
appearance  of  a  destructive  criticism  by  Gortner14  who  has  ex- 
amined the  parts  of  Chapter  III  of  Tower's  book,  in  which  he 
discusses  at  some  length  the  chemistry  of  the  pigments  in  Lep- 
tinotarsa  and  other  animals.  As  Gortner  has  shown,  this  dis- 
cussion, though  offered  with  every  show  of  confidence,  exhibits 
such  elementary  ignorance,  both  of  the  special  subject  and  of 
chemistry  in  general,  that  it  cannot  be  taken  into  serious  con- 
sideration. 

Some  observations  made  by  Dr.  W.  T.  Macdougal15  have 
also  been  interpreted  as  showing  the  actual  causation  of  genetic 
variation  by  chemical  treatment.  Of  these  perhaps  the  least  open 
to  objection  were  the  experiments  with  Raimannia  odorata,  a 
Patagonian  plant  closely  allied  to  Oenothera.  The  ovaries  were 
injected  with  various  substances  and  from  some  of  the  seeds 
which  subsequently  formed  in  them  a  remarkable  new  variety 
was  raised.  This  varying  or  mutational  form  was  strikingly 
different  from  the  parental  type,  with  which  it  was  not  con- 
nected by  any  intergradational  forms,  and  it  bred  true.  It  made 

11  As  to  the  interrelations  of  these  three  forms,  Tower  states  (1906,  p.  18)  that 
angustavittata,  which  he  reared  from  undecimlineata,  is  intermediate  between  it 
and  signaticollis.  Compare  Stal,  "Monogr.  des  Chrysomelides,"  1862,  p.  163;  and 
Jacoby,  Biol.  Centr.  Amer.  Celeopt.,  vi,  Pt.  i,  p.  234,  PI.  xiii,  fig.  20;  Tab.  41,  fig. 
15;  ibid.,  Suppl.,  p.  253.  All  these  forms  are  evidently  very  closely  related,  and 
the  delimitation  of  species  is  quite  arbitrary.  Jacoby  indeed  suggests  that  unde- 
cimlineata may  be  a  variety  of  decemlineata. 

14  Gortner,  Amer.  Nat.,  Dec.,  1911,  XLV,  p.  743. 

16  Mutations,  Variations,  and  Relationships  of  the  Oenotheras,  Carnegie  Institu- 
tion Publication  No.  81,  1907,  pp.  61-64. 


228  PROBLEMS  OF  GENETICS 

no  rosette,  growing  to  a  much  smaller  size  than  the  parent,  and 
was  totally  glabrous  instead  of  being  very  hairy  as  the  parental 
type  is.  I  was  shown  specimens  of  these  plants  by  the  kindness 
of  Dr.  Britton  in  the  Bronx  Park  Botanic  Garden  in  1907  and 
can  testify  to  their  very  remarkable  peculiarities.  They  had  a 
somewhat  weakly  look,  and  might  at  first  sight  be  thought  to  be 
a  pathological  product,  but  they  had  bred  true  for  several 
generations.  From  the  evidence,  however,  I  am  by  no  means 
satisfied  that  their  original  appearance  was  a  consequence  of 
the  treatment  applied.  This  treatment  was  of  a  most  miscel- 
laneous description.  Two  of  the  mutants  came  from  an  ovary 
which  had  been  treated  with  a  ten  per  cent,  sugar  solution.  Ten 
came  from  one  into  which  a  o.i  per  cent,  solution  of  calcium 
nitrate  had  been  injected.  One  was  from  a  capsule  which  "had 
been  exposed  to  the  action  of  a  radium  pencil."  Macdougal 
speaks  of  these  results  as  decisive,  but  clearly  before  such  evidence 
can  be  admitted  even  for  consideration  it  must  be  shown  by  con- 
trol experiments  that  the  individual  plants  which  threw  the 
mutant  were  themselves  breeding  true  in  ordinary  circumstances. 
Nothing  is  more  likely  than  that  the  mutant  was  an  ordinary 
recessive.  I  may  add  that  Mr.  R.  H.  Compton  made  a  number 
of  experiments  with  Raimannia  odorata,  raised  from  seeds  kindly 
given  me  by  Dr.  Britton,  injecting  the  ovaries  with  a  variety  of 
substances,  including  those  named  by  Macdougal;  but  though  a 
numerous  progeny  was  raised  from  the  ovaries  treated,  all  were 
normal.  Macdougal  relates  also  that  some  mutational  forms 
came  from  ovaries  of  Oenothera  Lamarckiana  exposed  to  radium 
pencils,  and  also  from  Oenothera  biennis  injected  with  zinc  sul- 
phate a  peculiar  mutant  was  raised,  but  taking  into  account  the 
frequency  of  these  occurrences  in  those  species,  he  very  properly 
regarded  this  evidence  as  of  doubtful  application.  In  a  later 
paper,16  however,  he  has  returned  to  the  subject  and  affirms  his 
conviction  that  the  appearance  of  a  mutant  among  seedlings 
raised  from  an  ovary  of  Oenothera  biennis  treated  with  zinc 
sulphate  was  really  a  consequence  of  the  injection,  saying  that 

16  Macdougal,  D.  T.,  "Alterations  in  Heredity  induced  by  Ovarial  Treat- 
ments," Bot.  Gaz.,  vol.  51,  1911,  p.  241. 


CAUSES  OF  GENETIC  VARIATION  229 

the  variation  previously  observed  in  the  species  was  afterwards 
shown  to  be  due  to  fungoid  disease.  The  circumstances  to  which 
he  mainly  points  in  support  of  his  view  is  that  the  mutation  bred 
true,  but  this  is  only  evidence  of  its  genetic  distinctness,  which 
may,  of  course,  be  admitted  by  those  who  remain  unconvinced 
as  to  the  original  cause  of  its  appearance.  He  adds  that  he  is 
making  similar  experiments  with  some  twenty  genera;  but  what 
is  more  urgently  needed  is  repeated  confirmation  of  the  original 
observation.  When  it  has  been  shown  that  this  mutation  can 
be  produced  with  any  regularity  from  a  plant  which  does  not 
otherwise  produce  it  on  normal  self -fertilisation,  the  enquiry 
may  be  profitably  extended  to  other  plants. 

A  curious  and  novel  experiment,  which  however,  led  ulti- 
mately to  a  negative  result,  was  made  by  F.  Payne.  Many  dis- 
cussions have  been  held  respecting  the  blindness  of  cave  animals. 
The  phenomenon  is  one  of  the  well-known  difficulties,  and  most 
of  us  would  admit  that  the  theory  of  evolution  by  the  natural 
selection  of  small  differences  does  not  offer  a  really  satisfying  ac- 
count of  it.  Those  who  believe  in  the  causation  of  such  modifica- 
tions by  environmental  influences  and  in  their  hereditary  trans- 
mission make,  of  course,  the  simple  suggestion  that  the  darkness 
is  the  cause  of  the  loss  of  sight,  and  that  disuse  has  led  to  the 
reduction  of  the  visual  organs.  Payne  bred  Drosophila  ampelo- 
phila,  the  pomace-fly  (which  is  easy  to  keep  in  confinement,  fed 
on  fermenting  bananas) ,  for  sixty-nine  generations  in  darkness. 
At  the  end  of  that  period  there  was  no  perceptible  change  in  the 
structure  of  the  eyes,  or  in  any  other  respect.  The  number  of 
generations  may  possibly  be  regarded  as  insufficient  to  prove 
anything,  but  comparing  them,  as  he  does,  with  the  generations 
of  mankind,  we  see  that  they  correspond  with  a  period  of  about 
two  thousand  years,  an  interval  far  longer  than  those  which 
many  writers  in  particular  cases  have  deemed  sufficient. 

In  his  first  paper  Payne  states  that,  though  no  structural 
difference  could  be  perceived,  the  flies  which  had  been  bred  in 
the  dark  reacted  less  readily  to  light  than  those  which  had  been 
reared  under  normal  conditions,  and  he  inclined  to  think  that 
the  treatment  had  thus  produced  a  definite  effect.  After  more 


230  PROBLEMS  OF  GENETICS 

careful  tests,  however,  he  withdrew  this  opinion.  It  proved 
that  both  individual  flies  and  individual  groups  of  flies,  both  of 
those  bred  in  the  light  and  of  those  bred  in  the  dark,  differed 
greatly  in  their  reactions,  which  were  measured  by  counting  the 
time  that  it  took  for  a  fly  to  travel  to  the  light  end  of  a  covered 
tube,  various  sources  of  error  being  eliminated.  He  found  further 
that  these  differences  of  behaviour  were  not  inherited  in  any 
simple  way,  but  he  is  disposed  to  attribute  them  to  accidental 
differences  in  the  nature  of  the  food,  an  account  which  seems 
probable  enough.17 

In  several  recent  publications  Blaringhem18  has  described 
the  origin  of  many  abnormal  forms  of  plants,  especially  of  maize, 
which  he  attributes  to  various  mutilations  practised  upon  the 
parents.  Respecting  these  the  same  difficulty  which  has  been 
expressed  in  other  cases  reappears,  that  before  drawing  any 
conclusion  as  to  the  value  of  such  evidence  we  require  to  know 
that  the  plants  treated  belong  to  a  really  pure  line,  which  if 
left  to  nature  in  the  ordinary  circumstances  of  its  life  in  that 
locality  would  have  had  normal  offspring.  Abnormalities  abound 
in  the  experience  of  everyone  who  examines  pans  of  seedlings 
of  almost  any  species  of  plant,  and  in  maize  they  are  well  known 
to  be  exceptionally  common.  Some  of  those  which  we  meet 
with  when  we  attempt  to  ripen  maize  in  this  country  are  very 
similar  to  those  which  Blaringhem  describes,  consisting  in  ir- 
regularities in  the  distribution  of  the  sexes,  in  the  shapes  of  the 
panicles,  etc.  Many  of  these  are  doubtless  imperfections  of 
development,  due  to  the  dullness  of  our  climate,  but  others  are 
presumably  genetic  and  would  recur  in  the  offspring  however 
treated.  If  some  one  working  in  a  climate  where  maize  could 
be  raised  in  perfection  would  repeat  these  experiments,  and  show 
that  a  strain  which  was  thoroughly  reliable  and  normal  in  its 
genetic  behaviour  did,  after  mutilation,  throw  the  miscellaneous 
types  observed  by  Blaringhem,  that  would  be  evidence  at  least 
that  the  development  of  the  seed  could  be  so  influenced  by 
injury  to  the  parental  tissues  that  its  properties  were  changed. 

"Payne,  Fernandus,  Biol.  Bull.,  XVIII,  1910,  p.  188,  and  ibid.,  XXI,  1911, 
p.  297. 

18  See  especially,  Mutation  et  Traumatismes,  Paris,  Felix  Alcan,  1908. 


CAUSES  OF  GENETIC  VARIATION  231 

Such  evidence  could  be  used  for  what  it  is  worth ;  but  pending  an 
inquiry  of  this  kind  I  am  disposed  to  regard  these  observations  of 
variation  following  on  parental  injury  as  suggestive  rather  than 
convincing. 

Some  evidence  of  a  remarkably  interesting  kind  has  been 
collected  by  J.  H.  Powers19  respecting  the  structure  and  habits 
of  Amblystoma  tigrinum,  which  led  him  to  the  conclusion  that 
striking  differences  in  the  form,  anatomy,  and  developmental 
processes  could  be  effected  directly  by  change  in  the  conditions 
of  life.  It  is  well  known  that  a  profusion  of  forms,  distinct  in 
various  degrees,  is  grouped  round  Amblystoma  tigrinum.  Some 
of  these  are  believed  to  be  geographically  isolated,  others  occur 
together  in  the  same  waters,  and,  as  usual,  authorities  have  dif- 
fered greatly  as  to  the  number  of  names  to  be  given.  These  forms 
were  studied  in  detail  by  Cope  who  described  them  in  the  Ba- 
trachia  of  North  America.  The  view  which  he  inclined  to  take 
was  that  the  individual  variations  of  Amblystoma  tigrinum  re- 
sulted from  variations  in  the  time  and  completeness  of  the 
metamorphosis,  and  these  were  regarded  as  due  to  external 
causes,  such  as  differences  in  season,  temperature,  and  geo- 
graphical conditions.  Powers,  however,  states  that  collecting 
within  a  radius  of  six  or  eight  miles  he  found  almost  if  not  quite 
the  whole  "gamut  of  recorded  variation  in  this  species."  Some, 
however,  as  he  states,  occurred  rarely  except  under  experimental 
conditions,  but  considerable  differences  in  temperature  were  not 
found  necessary  in  producing  them.  Every  year,  he  says,  he 
has  been  able  to  add  to  the  number  of  peculiar  types  found  in 
the  same  small  area  in  nature,  until  the  amount  of  natural 
variation  at  least  equals  that  seen  by  Cope  in  the  collections  of 
the  National  Museum  and  those  of  the  Philadelphia  Academy. 

Powers  states  that  his  observations  by  no  means  confirm 
Cope's  view  that  these  differences  are  in  the  main  referable  to 
variation  in  the  completeness  of  metamorphosis,  and  on  the 
contrary,  he  regards  metamorphosis  as  on  the  whole  a  levelling 
process,  tending  to  obliterate  diversity.  The  enormous  dif- 

19  J.  H.  Powers,  "Morphological  Variation  and  its  Causes  in  Amblystoma 
tigrinum."  Studies  from  the  Zoological  Laboratory.  The  University  of  Nebraska, 
No.  71,  1907. 


232  PROBLEMS  OF  GENETICS 

ferences  in  size  and  proportions  which  he  describes  can  only  be 
appreciated  by  reference  to  his  figures.  They  affect  almost  all 
features  of  bodily  organisation.  These  striking  differences  he 
looks  upon  as  brought  about  by  differences  in  nutrition,  "diversi- 
ties in  habitual  locomotion,"  and  diversity  in  the  age  at  which 
metamorphosis  occurs,  and  to  sexual  difference.  Apart  from 
sexual  difference  he  regards  the  chief  distinctions,  in  brief,  as 
"acquired  variations  of  the  larva." 

As  an  example  he  gives  the  great  elongation  of  some  of  the 
forms  as  "due  first  to  slow  growth,  second  to  the  free-swimming 
habit,  third  to  the  prolongation  of  larval  life,  and  finally  to  the 
assumption  of  sexual  maturity  as  males,"  either  in  the  branchiate 
or  non-branchiate  condition.  He  describes  the  rapid  growth  of 
some  and  the  slow  growth  of  others.  A  larva  of  intermediate 
type  may  grow  about  a  centimeter  a  month,  but  a  rapidly  growing 
specimen  may  grow  more  than  four  times  as  much.  The  slower 
rate  of  growth  may,  he  says,  be  induced  by  winter  feeding,  and 
other  treatment.20 

When,  however,  he  goes  on  to  describe  the  influences  which 
he  regards  as  exerted  by  the  habit  of  freely  swimming,  I  am  led 
to  wonder  whether  after  all  in  most  of  these  illustrations,  the 
primary  distinctions  are  not  in  reality  genetic.  "Specimens 
raised  in  the  same  aquarium  or  in  similar  aquaria,  side  by  side 
with  all  conditions  as  uniform  as  it  is  possible  to  make  them, 
seldom  fail  to  furnish  striking  examples  of  broad-headed,  short- 
bodied,  and  short- tailed  types  which  are  habitually  found  at  the 
bottom,  while  others,  slender  and  elongated,  are  free  swimmers, 
and  maintain  themselves  in  almost  as  continual  suspension  and 
motion  as  does  a  gold  fish."  Later,  again,  he  writes,  "Yet  despite 
the  uniformity  of  these  favourable  conditions,  the  larvae  soon 
began  to  split  up  into  two  noticeably  distinct  groups,  the  one  of 

20  In  connexion  with  this  case  I  would  refer  the  reader  to  some  remarkable 
observations  of  Dr.  T.  A.  Chapman  on  various  types  of  larvae  which  he  reared 
from  the  moth  Arctia  caja  (Ent.  Rec.,  IV,  1893,  p.  265,  and  following  parts).  From 
a  single  mother  he  raised  a  great  diversity  of  forms,  some  which  fed  up  rapidly 
and  passed  through  their  development  without  assuming  certain  stages,  and  others 
which  were,  as  he  called  them,  "laggards,"  moulting  more  times  than  their  brethren 
and  developing  at  a  much  slower  rate.  It  is  greatly  to  be  hoped  that  such  a  case 
may  be  critically  investigated  by  analytical  breeding. 


CAUSES  OF  GENETIC  VARIATION  233 

unusually  compact  proportions,  the  other  of  uniform  intermediate 
build,  such  is  most  commonly  met  with."  It  is  to  my  mind 
scarcely  possible  to  resist  the  inference  that,  though  there  may  be 
definite  responses  to  certain  conditions,  yet  the  chief  distinctions 
are  genetic,  and  that  it  is  these  distinctions  which  confer  the 
power  to  respond.  The  parts  respectively  played  by  cause  and 
effect  are  always  difficult  to  assign;  but  when  it  is  stated  that 
"a  weak-limbed,  long-bodied  and  long- tailed  animal  becomes 
well  nigh  perforce  an  undulatory  swimmer,  while  the  strong- 
limbed,  short-tailed,  heavy-bodied  specimen,  when  these  charac- 
teristics are  rapidly  forced  upon  it,  is,  under  certain  circum- 
stances, just  as  forcibly  induced  to  become  a  crawler,"  we  feel 
how  erroneous  any  estimates  of  causation  are  likely  to  be. 

One  of  the  most  remarkable  and  interesting  sections  of 
Powers'  paper  is  that  in  which  he  describes  the  differences  in 
bodily  structure  and  habits  which  he  attributes  to  cannibalism, 
and  the  whole  account  of  the  phenomena  should  be  read  in  the 
original.  It  appears  that  there  are  two  extremely  distinct 
types  of  larvae,  those  with  narrow  heads  and  slender  bodies 
which  live  for  the  most  part  on  small  Crustacea  such  as  Daphnias, 
and  those  with  huge  mouths  and  very  wide  heads,  which  dis- 
regard such  small  animals  altogether  and  live  on  amphibian 
larvae,  whether  of  their  own  or  other  species.  As  the  illustra- 
tions show,  the  differences  between  these  two  types  are  very 
great,  and  the  differences  in  instinct  and  behaviour  are  no  less. 
The  cannibals  take  no  heed  of  the  pelagic  Crustacea,  lying  slug- 
gishly at  the  bottom,  rousing  themselves  immediately  to  a 
violent  attack  on  the  larger  living  things  which  approach  them. 
Nothing  but  the  most  incontrovertible  evidence  based  on  abun- 
dant control  experiments  should  convince  us  that  such  differences 
are  not  primarily  genetic,  and  in  the  present  state  of  knowledge 
I  incline  to  think  that  the  families  really  consist  of  individuals 
which  are  ready  to  assume  the  cannibal  habit  if  opportunity 
offers,  and  others  which  are  congenitally  incapable  of  it.  It  may 
readily  be  that  if  all  chance  of  cannibal  diet  be  excluded,  the 
full  development  of  the  wide  head  and  mouth,  or  the  other 
peculiarities,  would  never  become  pronounced, but  I  doubt  whether 
such  change  could  be  induced  in  any  individual  taken  at  random. 


CHAPTER  XI. 

STERILITY  OF  HYBRIDS.    CONCLUDING  REMARKS. 

WHEN  we  consider  the  bearing  of  recent  discoveries  on  those 
comprehensive  schemes  of  evolution  with  which  we  were  formerly 
satisfied,  we  find  that  certain  details  of  the  process  are  more  easy 
to  imagine.  We  readily  now  understand  how  varieties  once 
formed,  can  persist,  but  at  the  same  time  difficulties  hitherto 
faced  with  complacency  become  formidable  in  the  light  of  the 
new  knowledge.  So  generally  is  this  admitted  by  those  familiar 
with  modern  genetic  research  that  most  are  rightly  inclined  to 
postpone  the  discussion.  The  premisses,  indeed,  on  which  such 
a  discussion  must  be  based  are  almost  wholly  wanting. 

The  difficulties  to  which  I  chiefly  refer  are  not  those  created 
by  the  phenomena  of  adaptation,  though  they  are  serious  enough. 
In  treating  of  that  subject  I*  have  felt  obliged  to  express  scepti- 
cism as  to  the  validity  of  nearly  all  the  new  evidence  for  the 
transmission  of  acquired  characters.  At  the  present  time  the 
utmost  we  are  bound  to  accept  is  the  proof  that  (i)  in  some 
parthenogenetic  forms  variations,  or  perhaps  we  may  say  mal- 
formations, produced  in  response  to  special  conditions,  recur  in 
one  or  perhaps  two  generations  asexually  produced  after  removal 
to  other  conditions.  (2)  That  violent  maltreatment  may  in  rare 
instances  so  affect  the  germ-cells  contained  in  the  parents  as  to 
cause  the  individuals  resulting  from  the  fertilisation  of  those 
cells  to  exhibit  an  arrest  of  development  similar  to  that  which 
their  parents  underwent. 

I  do  not  doubt  that  evidence  of  this  type  will  be  greatly 
extended.  As  a  contribution  to  genetic  physiology  these  facts 
are  very  important  and  interesting,  but  I  cannot  think  that  any 
one,  on  reflexion,  will  feel  encouraged  by  such  indications  to 
revive  old  beliefs  in  the  direct  origin  of  adaptations. 

In  these  respects  we  are  simply  left  where  we  were.  The 
force  of  objections  based  upon  the  existence  of  adaptative 

234 


STERILITY  OF  HYBRIDS  235 

mechanisms  is  no  greater  than  it  has  always  been.  On  the  con- 
trary the  fact  that  variations  can  now  so  generally  be  recognized 
as  definite  is  some  alleviation  of  the  difficulty.  We  can  moreover 
disabuse  ourselves  of  the  notion  that  for  all  characters  which  are 
definite  or  fixed,  some  utilitarian  rationale  may  be  presumed. 
Upon  that  point  the  study  of  variation  has  provided  a  perfectly 
clear  answer. 

In  frankly  recognizing  that  the  fixity  of  characters  in  general 
need  not  connote  usefulness  to  their  possessors  we  deliver  our- 
selves of  a  distracting  pre-occupation  and  prepare  our  minds  for 
an  investigation  of  the  properties  of  living  organisms  in  the 
same  spirit  as  that  in  which  the  chemist  and  the  physicist 
examine  the  properties  of  unorganized  materials.  The  creature 
persists  not  merely  by  virtue  of  its  characteristics  but  in  spite 
of  them,  and  the  fact  of  its  persistence  proves  no  more  than 
that  on  the  whole  the  balance  of  its  properties  leaves  something 
in  its  favour. 

It  may  be  noted  by  the  way  that  the  fact  that  the  structures 
of  living  things  are  on  the  whole  adaptative  was  not  always 
obvious.  Though  to  naturalists  of  this  generation  it  is  a  truism, 
we  have  only  to  turn  to  Buffon  to  find  that  in  his  philosophy  of 
nature  it  played  no  essential  part.  The  passage  in  which  Buffon 
describes  what  he  regards  as  the  forlorn  and  degraded  condition 
of  the  Woodpecker  is  well  known.  We  have  come  to  think  of 
the  Woodpecker  as  a  capital  example  of  adaptation  to  the  mode 
of  life;  but  Buffon  after  enumerating  the  hard  features  of  the 
bird's  existence,  forced  to  earn  its  living  by  piercing  the  bark  of 
trees  in  an  attitude  of  perpetual  constraint,  remarks1  "  Tel  est 
1'instinct  etroit  et  grossier  d'un  oiseau  born£  a  une  vie  triste  et 
chetive.  II  a  regu  de  la  Nature  des  organes  et  des  instrumens 
appropries  a  cette  destinee  ou  plutot  il  tient  cette  destinee  meme 
des  organes  avec  lesquels  il  est  ne"  (my  italics).  His  reflexions 
on  the  Stilt  (Himantopus)  read  even  more  strangely  to  us, 
accustomed  as  we  are  to  see  in  the  prodigious  length  and  thinness 
of  the  shanks  and  in  the  other  features  of  its  organisation  pal- 
pable adaptations  to  a  wading  life.  For  Buffon,  however,  this 

1  Buffon,  Hist.  Nat.,  Oiseaux,  1780,  VII,  p.  3. 


236  PROBLEMS  OF  GENETICS 

curious  bird  seemed  a  poor,  neglected  production,  extravagant 
in  its  disproportions,  one  of  the  misfits  of  creation,  left  as  a 
shadow  in  the  picture  composed  of  nature's  more  successful 
efforts.2  This  theme  he  develops  at  some  length,  being  evidently 
well  pleased  with  the  idea. 

Our  way  of  regarding  these  things  is  doubtless  sounder  and 
more  fruitful  than  Buffon's,  but  it  is  well  to  remember  that  what 
seems  so  obvious  to  us  looked  quite  differently  to  other  excellent 
observers;  and  stupid  as  it  may  have  been  to  have  overlooked 
plain  examples  of  adaptation,  it  is  a  far  worse  mistake  to  see 
adaptation  everywhere.  I  do  not  seek  to  minimise  the  real 
and  permanent  difficulty  which  the  existence  of  adaptations 
creates,  but  by  the  suggestion  that  all  normal  specific  differences 
are  adaptational  that  difficulty  was  quite  gratuitously  increased. 

In  these  respects  it  may  be  claimed  that  progress  has  been 
made,  even  if  that  progress  seem  outwardly  of  small  account. 

But  all  constructive  theories  of  evolution  have  been  built  on 
the  understanding  that  what  we  know  of  the  relation  of  varieties 
to  species  justifies  the  assumption  that  the  one  phenomenon  is  a 
phase  of  the  other,  and  that  each  species  arises  or  has  arisen 
from  another  species  either  by  one  or  several  genetic  steps.  In 
the  varieties  we  have  accustomed  ourselves  to  think  that  we  see 
those  steps.  We  still  know  little  enough  of  the  mode  of  occur- 
rence of  variation,  but  we  do  begin  to  know  something,  and  if  we 
ask  ourselves  whether  our  knowledge,  such  as  it  is,  conforms  at 
all  readily  with  our  former  expectations,  we  cannot  with  any 
confidence  assert  that  it  does.  Among  the  plants  and  animals 
genetically  investigated  are  many  illustrations  of  very  striking 
and  distinct  varieties.  Many  of  these  might  readily  enough  be 
accepted  as  species  by  even  the  most  exacting  systematists,  and 
not  a  few  have  been  so  treated  in  classification;  but  when  we 
have  examined  their  relationship  to  each  other  we  feel  not  merely 
that  they  are  not  species  in  any  strict  sense  but  that  the  dis- 
tinctions they  present  cannot  be  regarded  as  stages  in  the  direc- 
tion of  specific  difference.  Complete  fertility  of  the  results  of 
inter-crossing  is  and  I  think  must  rightly  be  regarded  as  incon- 

2  ibid.,  viii,  p.  115. 


STERILITY  OF  HYBRIDS  237 

sistent  with  actual  specific  difference;  and  of  variations  leading 
to  that  consequence  no  clear  indication  has  yet  been  found. 
As  an  example  of  possible  exceptions  mention  should  perhaps  be 
made  of  the  case  of  a  giant  form  of  Primula  sinensis  investigated 
by  Keeble.3  It  arose  from  a  "  Star  "  Primula  of  normal  size, 
and  though  fertile  with  its  own  pollen  all  attempts  to  fertilise 
it  with  the  pollen  of  other  forms  failed.  Miss  Pellew,  who  did 
these  fertilisations,  tells  me  that  very  extensive  trials  were  made, 
and  repeated  in  several  seasons.  Ultimately  two  plants  were 
raised  from  it  fertilised  with  a  plant  of  the  strain  from  which  it 
sprang,  and  these  proved  sterile.  In  the  light  of  modern  expe- 
rience the  significance  of  such  isolated  instances  is  doubtful. 

All  the  strains  known  as  "  Giants  "  are,  as  Messrs.  Sutton 
have  always  found,  more  or  less  sterile,  and  their  sterility  is 
presumably  due  to  some  negative  defect. 

In  regard  to  the  fertility  of  Primula  species  there  are  several 
paradoxes.  For  example  the  long-styled  varieties,  apart  from 
giants,  are  fertile  with  their  own  pollen,  and  for  many  years 
short-styled  plants  have  not  been  used  in  most  strains.  Auriculas 
and  Polyanthuses,  on  the  contrary,  are  generally  if  not  always 
bred  from  short-styled  plants,  as  the  florists  have  decided  that 
the  long-styled  are  inadmissible.  Mr.  R.  P.  Gregory  tells  me 
that,  though  most  strains  of  P.  sinensis  give  seed  enough 
when  only  long-styled  plants  are  used,  he  finds  nevertheless 
that  when  a  "  legitimate  "  union  is  made  the  amount  of  seed 
usually  increases  much  as  Darwin  observed.  Darwin's  state- 
ment that  plants  of  "  illegitimate  "  origin  are  less  fertile  than  the 
"  legitimately  "  raised  plants  is  also  in  general  confirmed  by  his 
experience.  To  this  rule  there  were  some  marked  exceptions  in 
strains  derived  from  long-styled  plants,  which  though  illegitimate 
showed  a  high  degree  of  fertility,  but  illegitimate  unions  between 
short-styled  plants  always  produced  comparatively  sterile  off- 
spring. I  have  no  records  of  the  behavior  of  Auriculas  and 
Polyanthuses.  It  would  be  interesting  to  know  whether  among 
them  pure  strains  of  short-styled  plants  (dominants)  have 
appeared,  and,  if  so,  how  their  fertility  is  affected.  Without 

•Keeble,  Jour.  Gen.,  1912,  II,  p.  173. 


238  PROBLEMS  OF  GENETICS 

much  more  critical  data  I  suppose  no  one  would  nowadays  be 
inclined  to  follow  Darwin  in  instituting  a  comparison  between 
the  sterility  of  hybrids  and  that  of  illegitimately  raised  plants  of 
heterostyle  species.4  It  is  even  difficult  to  imagine  any  essential 
resemblance  between  these  two  phenomena,  nor  has  evidence 
ever  been  produced  to  show  that  illegitimately  raised  plants 
have  bad  pollen  grains,  which  is  the  usual  symptom  of  sterility 
in  hybrid  plants  and  the  consequence,  as  we  believe,  of  failure 
of  some  essential  division  in  the  process  of  maturation. 

The  difficulty  that  we  have  no  knowledge  of  the  contemporary 
origin  of  forms,  from  a  common  stock,  which  when  crossed  together 
give  a  sterile  product,  is  one  of  the  objections  constantly  and 
prominently  adduced  from  the  time  of  the  first  promulgation  of 
evolutionary  ideas.  In  the  light  of  recent  work  the  objection 
has  gathered  strength.  Why,  if  we  are  able  to  produce  instances 
of  variation  colourably  simulating  specific  difference  in  almost 
all  other  respects,  do  we  never  find  an  original  appearance  of  this 
most  widely  spread  of  all  specific  characteristics?  No  doubt  all 
breeders  know  that  sterile  animals  and  plants  occasionally  appear 
in  their  cultures,  but  it  is  more  in  accordance  with  probability 
that  the  sterility  in  these  sporadic  instances  should  be  regarded 
as  due  to  defect  than  that  it  should  be  thought  comparable 
with  that  of  the  sterile  hybrids.  For  their  sterility  must,  by  all 
analogy  with  results  elsewhere  seen,  be  attributed  not  to  the 
absence  of  something,  but  to  the  presence  and  operation  of 
complementary  factors  leading  to  the  production  of  inhibition 
of  division;  and  consistently  with  that  interpretation,  we  find 
that  when  from  a  partially  sterile  hybrid  comparatively  fertile 
offspring  can  be  raised,  their  comparative  fertility  continues  in 
the  posterity  generally  if  not  always  without  diminution.  The 
distinction  between  these  several  kinds  of  sterility  was  of  course 
not  understood  in  Darwin's  time.  The  comparison,  for  example, 
which  he  instituted5  between  the  sterility  of  "  contabescent  " 
anthers  and  that  of  hybrids  no  longer  holds,  for  at  least  in  those 
cases  in  which  the  nature  of  contabescent  anthers  have  been 
genetically  investigated  (Sweet  Pea,  Tropaeolum)  they  proved 

4  Animals  and  Plants,  ed.  i,  1868,  II,  pp.  180-5. 
B  Animals  and  Plants,  ed.  i,  1868,  II,  p.  165. 


STERILITY  OF  HYBRIDS  239 

to  be  a  simple  recessive  character.  Nor  can  we  now  easily 
suppose  that  the  attempt  there  made  by  Darwin  to  suggest 
resemblance  between  the  sterility  produced  by  unnatural  condi- 
tions and  that  of  hybrids  has  any  physiological  justification. 

In  regarding  the  power  to  produce  a  sterile  or  partially 
sterile  hybrid  as  a  distinction  in  kind,  of  a  nature  other  than 
those  which  we  perceive  among  our  varieties,  I  am  aware  that 
I  am  laying  stress  on  an  impression  which  may  hereafter  prove 
false.  The  distinction  nevertheless  is  so  striking  and  so  con- 
tinually before  the  eyes  of  a  practical  breeder  that  he  can 
scarcely  avoid  the  inference  that  when  he  meets  a  considerable 
degree  of  sterility  in  a  cross-bred  he  is  dealing  with  something 
belonging  to  a  distinct  category,  and  not  merely  a  varietal  feature 
of  an  exceptional  kind. 

Besides  the  sterility  of  hybrids  appeal  has  often  been  made  to 
the  phenomenon  of  incompatibility,  in  its  several  stages  of 
completeness,  as  distinguishing  species.  No  one  doubts  that  in- 
compatibility may  arise  from  a  variety  of  causes  of  most  diverse 
degrees  of  importance,  but  though  sometimes  referred  to  as  an 
extreme  case  of  interspecific  sterility,  it  is  really  a  very  different 
matter.  In  regard  to  one  phase  of  this  incompatibility,  that 
associated  with  self -sterility,  some  progress  has  been  made,  and 
we  are  not  wholly  without  experimental  evidence  of  its  being 
within  the  range  of  contemporary  variation. 

Given  the  outline  of  Mendelian  teaching  as  to  gametic  dif- 
ferentiation and  the  classification  of  individuals  in  a  mixed 
population,  it  seemed  highly  probable  that  what  we  call  self- 
sterility  must  mean  that  the  species  really  consisted  of  classes, 
some  of  which  are  capable  of  interbreeding  with  others  while 
others  are  not.  According  to  the  received  account  every  indi- 
vidual, though  incapable  of  fertilising  itself,  was  supposed  to  be 
able  both  to  fertilise  and  to  be  fertilised  by  any  other  individual. 
This  notion  has  always  seemed  to  me  a  self-evident  absurdity, 
for  it  would  imply  that  there  can  be  as  many  categories  as 
individuals.  Such  experiments,  however,  as  I  made  did  cer- 
tainly give  results  consistent  with  that  belief.  I  first  tried 
Cinerarias,  which  are  usually  self-sterile,  but  I  found  no  in- 


c4o  PROBLEMS  OF  GENETICS 

compatible  pairs  of  plants.  Whether  I  was  deceived  by  the 
consequences  of  apogamy,  or  whether  the  pollen  of  certain  plants 
may  belong  to  more  than  one  class  I  do  not  know.  The  results 
were  confused  in  various  ways.  Usually  the  self-fertilised  plants 
set  little  or  nothing,  and  cross-fertilised  they  set  fully  with  such 
uniformity  that  the  few  failures  could  plausibly  be  attributed  to 
mistakes  in  manipulation  or  to  other  extraneous  causes.  Later 
de  Vries  announced6  (without  giving  particulars)  that  he  had 
proved  the  existence  of  such  classes  in  Linaria  vulgaris;  but  on 
making  experiments  with  that  species  I  again  got  no  positive 
results,  and  I  came  to  the  conclusion  that  in  spite  of  inherent 
improbability  the  conventional  belief  must  be  substantially  true. 
At  last,  however,  the  work  of  Correns,  lately  published,7  does 
definitely  show  that  in  one  species,  Cardamine  pratensis,  classes  of 
individuals  exist  such  that  individuals  of  the  same  class  are 
incapable  of  fertilising  themselves  or  each  other,  but  fertilisation 
made  between  the  classes  is  usually  completely  effective.  Many 
complications  were  encountered  and  some  contradictory  evidence 
is  recorded,  but  the  general  bearing  of  the  results  was  positive 
and  indubitable. 

We  know  far  too  little  of  this  phenomenon  as  yet  to  be  able 
to  understand  its  significance,  but  I  suppose  we  may  anticipate 
with  some  confidence  that  it  will  be  found  to  be  a  manifestation 
of  dissimilarity  between  the  male  and  female  gametes  of  the 
same  individual,  comparable  with  that  first  seen  in  the  Stocks 
(Matthiola)  which  throw  doubles — a  state  of  things  in  all  likeli- 
hood to  be  found  widely  spread  among  hermaphrodite  organisms. 
Whether  the  incompatibility  between  species  is  to  be  associated 
with  that  of  the  self-steriles  also  cannot  be  positively  asserted, 
though  it  seems  not  unreasonable  to  expect  that  such  an  associa- 
tion will  be  discovered. 

The  case  of  the  apple  and  the  pear  is  an  impressive  illustration 
of  this  possibility.  The  two  species  are  of  course  exceedingly 
alike  in  all  outward  respects,  but  nevertheless  the  pollen  of  each 
is  entirely  without  effect  on  the  other.  Presumably  we  should 

6  Species  and  Varieties,  1905,  p.  471. 

7  Correns,  Festschr.  med.-nat.  Ges.  zur  84  VersammL  Deutsch.  Naturf.  u.  Aertze. 
Munster  i.  W.,  1912. 


STERILITY  OF  HYBRIDS  241 

interpret  this  fact  as  meaning  not  so  much  that  the  apple  and 
the  pear  are  in  reality  very  wide  apart,  but  rather  that  either, 
each  is  lacking  in  one  of  two  complementary  elements,  or  that 
each  possesses  a  factor  with  an  inhibitory  effect.  Their  incom- 
patibility may  well  be  of  the  same  nature  as  that  of  the  classes 
in  Cardamine  pratensis. 

Returning  now  to  the  problem  of  inter-specific  sterility;  we 
note,  as  I  have  said,  the  absence  of  contemporary  evidence  that 
variation  can  confer  on  a  variety  the  power  to  form  a  sterile 
hybrid  with  the  parent  species.  The  considerations  based  on 
this  want  of  evidence  have  for  a  long  while  been  familiar  to  all 
who  have  discussed  evolutionary  theories,  and  it  is  worth  observ- 
ing the  exact  reason  why  the  difficulty  strikes  us  now  with  a  new 
and  special  force.  In  pre-Mendelian  times  all  that  was  known 
was  that  some  forms  could  freely  interbreed  without  diminution 
of  fertility  in  the  product,  while  others  could  not.  But  now  we 
find  that,  by  virtue  of  segregation,  from  one  and  the  same  pair 
of  parents,  or  even,  in  the  case  of  hermaphrodites,  from  one  and 
the  same  individual,  offspring  commonly  arises  howing  among 
themselves  exactly  such  differences  as  distinguish  species — and 
very  good  species  too.  This  we  see  happening  again  and  again. 
But  to  forms  capable  of  arising  as  brethren  in  one  family  the 
title  species  has  never  been  meant  to  apply,  and  if  we  are  going 
to  use  the  term  in  application  to  fraternal  groups  we  must 
definitely  recognise  that  by  "  specific  "  difference  is  to  be  under- 
stood simply  difference,  without  any  immediate  or  even  ulterior 
physiological  limitation  whatever.  Naturally,  therefore,  we  begin 
to  think  of  the  appearance  of  sterility  in  crosses  as  something 
apart,  and  as  a  manifestation  which  distinguishes  certain  kinds 
of  unions  in  a  very  special  way. 

I  am  perfectly  aware  that  there  are  gradations  in  the  sterility 
of  hybrids  as  in  every  other  characteristic  upon  which  it  has  been 
proposed  to  base  specific  definitions;  but,  as  also  so  often  happens 
in  the  matter  of  defining  intergrading  categories,  the  difficulty 
in  practice  is  not  often  such  as  to  lead  to  actual  ambiguity.  I 
am  speaking  of  course  of  those  examples  which  are  amenable  to 
genetic  experiment.  As  to  the  rest  there  is  complete  and  perma- 

17 


242  PROBLEMS  OF  GENETICS 

nent  uncertainty.  But  the  experience  of  the  practical  breeder 
does,  I  think,  on  the  whole,  support  the  contention  to  which 
systematists  have  so  steadily  clung  under  all  the  assaults  of 
evolutionary  philosophers,  that,  though  we  cannot  strictly  define 
species,  they  yet  have  properties  which  varieties  have  not,  and 
that  the  distinction  is  not  merely  a  matter  of  degree. 

The  first  step  is  to  discover  the  nature  of  the  factors  which  by 
their  complementary  action  inhibit  the  critical  divisions  and  so 
cause  the  sterility  of  the  hybrid.  Thus  expressed,  we  see  the 
problem  of  inter-specific  sterility  in  its  right  place;  and  the 
question  why  we  do  not  now  find  contemporary  instances  of 
varieties  lately  arisen  in  domestication,  which  when  crossed  back 
with  their  parents,  or  with  their  coderivatives,  can  produce 
sterile  products,  is  perceived  to  be  only  a  special  case  of  a  problem 
which  in  its  more  general  form  is  that  of  the  origin  of  new  and 
additional  factors. 

For  the  requisite  evidence  no  comprehensive  search  has  been 
made,  but  perhaps  it  will  yet  be  found.  All  that  we  can  say  at 
the  present  time  is  that  the  incidence  both  of  hybrid  sterility, 
and  of  incompatibility  also,  is  most  capricious;  and  provided 
that  two  forms  have  such  features  in  common  that  a  cross  between 
them  seems  not  altogether  out  of  the  question,  no  one  can  predict 
without  experiment  whether  such  a  cross  is  feasible,  and  if 
feasible  whether  the  product  will  be  fertile,  or  sterile  more  or 
less  completely.  For  instance,  though  probably  all  the  British 
and  some  Foreign  Finches  (Fringillidae)  have  been  crossed 
together,  and  some  of  these  crosses,  as  for  instance,  the  various 
Canary-mules  have  been  made  in  thousands,  I  believe  no  quite 
clear  example  of  a  fertile  hybrid  can  be  produced.  Many  species 
of  Anatidae  cross  readily  and  produce  fertile  hybrids :  others  give 
results  uniformly  sterile.  Though  most  of  the  Equidae  can  be 
crossed  and  some  of  the  hybrids  are  among  the  commonest  of 
domesticated  animals  there  is  no  certain  record  of  a  fertile  mule. 
Among  the  Canidae  the  dogs,  wolves  and  jackals  all  give  fertile 
hybrids,  but  there  is  no  clearly  authenticated  instance  of  a  cross 
between  any  of  these  forms  and  the  European  fox.  In  spite  of 
their  close  anatomical  resemblance  it  is  doubtful  if  the  rabbit 


STERILITY  OF  HYBRIDS  243 

and  the  hare  have  ever  interbred.  Many  of  the  wild  species  of 
Bos  have  been  crossed  and  recrossed  both  with  each  other  and 
with  many  domesticated  races,  but  I  understand  that  no  cross 
with  the  Indian  buffalo  (Bos  bubalus)  has  yet  been  successful 
even  in  producing  a  live  calf.8  In  the  genus  Primula  many 
hybrids  are  known  and  several  of  them  occur  in  nature,  but 
hitherto  no  certain  hybrid  between  P.  sinensis  and  any  other 
species  has  been  made,  in  spite  of  repeated  attempts. 

In  Nicotiana  many — doubtless  all — the  various  forms  of  N. 
tabacum  can  be  crossed  together  without  diminution  of  fertility, 
though  some  are  very  distinct  in  appearance,  but  crosses  between 
tabacum  and  sylvestris  are  highly  sterile  (in  my  experience  totally 
sterile9) ,  though  the  distinctions  between  them  are  not  to  outward 
observation  nearly  so  great  as  those  which  can  be  found  between 
the  various  races  of  Primula  sinensis. 

Recently  some  remarkable  experiments  bearing  closely  on 
these  questions  have  been  published  by  F.  Rosen.10  They  con- 
cern the  forms  of  Erophila  (Draba)  verna,  celebrated  in  the  history 
of  evolutionary  theory  as  the  plants  especially  chosen  by  Alexis 
Jordan  for  the  exposition  of  his  views  on  these  subjects. 

The  "  species  "  contains  a  profusion  of  forms  dissimilar  in 
many  structural  characters,  such  as  the  size  and  shape  of  leaves, 
flowers,  fruits,  etc.  Of  these  forms  many  grow  in  association. 
Jordan  found,  on  experiment,  that  each,  to  the  number  of  some 
two  hundred,  bred  true,  and  that  therefore,  the  conventional 
assumption  that  polymorphism  of  this  kind  must  mean  great 
contemporary  variability  had  no  foundation  in  fact.  So  far 

8  This  is  a  case  of  a  somewhat  different  order  and  I  mention  it  partly  for  that 
reason  as  an  illustration  of  the  complexity  which  such  negative  instances  may 
present.     The  difficulty  is  that  though  the  buffalo  and  the  zebu  can  breed  together, 
the  foetus  is  too  large  to  be  born  alive.     (See  Ackermann  Ber.  d.  Ver.  f.  Naturk., 
Kassel,  1898,  p.  69.     Prof.  S.  Nathusius,  of  Halle,  who  has  great  experience  in 
crossing  Bovidae,  tells  me  that  he  has  always  failed  to  cross  the  buffalo  with 
other  species.) 

9  In  a  paper  to  be  published  in  the  Report  of  the  Genetic  Conference,  Paris, 
1911,  Bellair  states  that  he  obtained  some  partially  fertile  hybrids  in  the  cross 
N.  sylvestris  X  tabacum.     As  to  the  various  degrees  of  sterility  in  hybrids  between 
Nicotiana  species  see  Lock,  R.  H.,  Ann.  Roy.  Bot.  Gardens.     Peradeniya,  IV,  1909, 

P-  195- 

10  Beitrage  zur  Biol.  der  Pflanzen.,  X,  1911,  p.  379. 


244  PROBLEMS  OF  GENETICS 

indeed  is  the  evidence  from  favouring  the  belief  that  such  forms 
are  in  any  way  transitional  or  indeterminate,  that,  as  is  well 
known,  Jordan  used  it  with  every  plausibility  to  support  the 
doctrine  of  the  fixity  of  species.  To  certain  aspects  of  Jordan's 
work  we  will  return  later  in  this  chapter,  but  the  matter  is  in 
the  present  connection  of  especial  interest  for  the  reason  that 
Rosen  has  lately  found  by  experiment  that  some  of  these  pre- 
sumably very  closely  allied  forms,  crossed  together,  gave  hybrids 
more  or  less  sterile.  In  the  case  of  the  offspring  of  one  pair  of 
forms  only  (E.  cochleata  and  stricta)  was  the  fertility  undi- 
minished,  and  the  various  degrees  of  sterility  found  in  the 
other  crosses  ranged  up  to  the  extreme  infertility  of  the  hybrids 
between  E.  stricta  X  data.  From  this  cross  ten  plants  were 
bred.  Of  these  the  four  strongest  were  chosen  to  breed  from, 
but  two  of  the  four  proved  totally  sterile ;  one  had  only  bad  seeds ; 
and  from  the  fourth  a  single  seedling  was  raised  which  in  its 
turn  proved  to  be  sterile.  From  the  less  sterile  hybrids  F2 
families  were  raised,  with  the  usual  experience  that  in  this  and 
subsequent  generations  the  sterility  diminished  among  extracted 
forms,  new  and  true-breeding  types  with  complete  fertility  being 
thus  derived  from  the  original  cross.11 

The  production  of  sterility  as  a  consequence  of  crossing 
plants  so  nearly  approaching  each  other  as  these  Erophila 
"  species  "  do  is  not  a  little  interesting,  and  the  fact  well  ex- 
emplifies the  futility  of  the  various  attempts  to  frame  general 
expressions  as  to  specific  properties  or  behaviour.  Commenting 
on  his  results  Rosen  argues  that  the  polymorphic  group  commonly 
called  by  systematists  Erophila  (Drabd)  verna  may  now  be 
regarded  as  having  arisen  by  crossing,  as  did  his  own  types  men- 
tioned above.  The  question,  however,  what  species  were  the 
original  progenitors  of  the  group  cannot  be  answered.  Rosen 
considers  that  no  form  which  he  knows  satisfies  the  requirements, 

11  One  very  peculiar  feature  was  observed,  namely,  that  all  the  new  forms  in  Fa 
which  were  bred  from  came  true.  As  I  understand,  this  statement  applied  to  five 
such  new  types,  and  they  were  represented  by  76  individuals  in  F3,  but  further 
details  on  this  point  are  desirable.  Another  curious  fact  was  observed,  namely 
that  one  of  the  Fi  forms  (cochleata  X  radiata)  when  fertilised  by  cochleata  gave  a 
highly  polymorphic  family,  but  fertilised  by  radiata  the  resulting  offspring  were 
almost  uniform. 


STERILITY  OF  HYBRIDS  245 

and  that  it  or  they  must  be  supposed  to  be  lost.  This  conclusion 
will  recall  the  similar  problem  raised  by  the  Oenothera  mutants 
(Chap.  V);  and  unsatisfactory  as  it  may  be  to  have  recourse 
to  such  hypotheses  we  must  remember  the  possibility  that  as  a 
consequence  of  hybridisation,  subsequent  segregation  and  re- 
combination of  factors,  species  may  have  thus  actually,  as  we 
may  say,  exploded,  and  left  nothing  but  a  polymorphic  group  of 
miscellaneous  types  to  represent  them  in  posterity.  If  this  way 
of  regarding  the  phenomena  be  a  true  one,  the  sterility  now  seen 
when  some  of  the  group  are  re-crossed,  becomes  analogous  to 
that  "  reversion  or  crossing  "  which  we  now  so  well  understand 
to  be  a  consequence  of  the  recombination  of  characters  separated 
at  some  previous  point  in  the  history  of  descent.  In  the  partial 
sterility  of  the  contemporary  hybrid  we  see  this  character  re- 
appearing, formed  now  as  it  was  on  the  occasion  of  the  original 
cross,  by  the  meeting  of  complementary  factors. 

Another  case  that  may  be  mentioned  in  this  connection  is 
that  of  the  crosses  between  various  culinary  peas  (Pisum  sativum) 
and  a  peculiar  form  found  by  Mr.  Arthur  Sutton  growing  os- 
tensibly in  a  wild  state  in  Palestine.  This  Palestine  Pea  is  low 
growing,  rarely  reaching  18  inches.  It  is  in  general  appearance 
like  a  small  and  poorly  grown  field  pea.  The  stems  are  thin  and 
rather  hard.  The  most  obvious  differences  which  distinguish 
this  from  other  field  peas  are  the  marked  serration  of  the  stipules, 
and  the  development  of  pith  in  the  pods.  Such  pith  is  often 
present  in  the  pods  of  peas  more  or  less,  but  in  the  Palestines  it 
is  so  strongly  developed  as  almost  to  form  a  lomentum.  Curi- 
ously enough,  though  the  flowers  are  purple  much  as  those  of 
ordinary  field  peas,  there  is  no  coloured  spot  in  the  axils.  On 
the  other  hand,  the  stems  have  coloured  stripes  running  up 
from  the  axils.  Though  this  plant  differs  so  little  from  domes- 
ticated peas,  all  crosses  with  them  either  failed,  or  produced 
hybrids  quite  or  almost  quite  sterile.  This  was  Mr.  Sutton's 
experience,  and  on  repeating  the  experiments  with  material 
kindly  given  by  him  I  found  the  same  result.12 

In  a  large  series  of  crosses  some  seeds  died  or  gave  rise  to 

u  I  also  had  a  few  Fi  seeds  given  me  by  Mr.  R.  H.  Lock. 


246  PROBLEMS  OF  GENETICS 

feeble  plants.  Of  the  plants  which  lived,  few  gave  any  seed. 
The  seed,  however,  that  was  obtained  from  FI  plants  grew  well 
enough,  and  the  Fz  plants  proved,  as  often  in  such  cases,  fertile. 
In  these,  indeed,  no  sign  of  sterility  was  noticeable.  The  experi- 
ment is  being  repeated  in  various  ways,  for,  as  the  genetic 
behaviour  of  peas  is  comparatively  well  known,  the  subject  is  an 
exceptionally  favourable  one  for  these  investigations. 

Such  an  example  shows  the  confusion  produced  the  moment 
we  attempt  to  harmonize  conceptions  of  specific  difference  with 
results  attained  by  experimental  methods.  It  has  been  usual 
to  regard  the  field  pea  (P.  arvense)  as  a  species  distinct  from  the 
edible  pea  (P.  sativum).  De  Candolle  and  others  regard  the 
field  pea  as  derived  from  a  form  wild  in  Italy,  but  the  origin  of 
the  edible  pea  is  considered  to  be  unknown.  From  breeding 
experiments  we  find  no  sterility  whatever  in  the  crosses  between 
the  various  arvense  and  sativum  types,  nor  in  the  crosses  made 
between  them  and  several  other  peculiar  types  from  various 
countries;  whereas  this  Palestine  Pea,  which  only  differs  from  a 
small  arvense  in  what  might  have  been  thought  trivial  characters,13 
either  fails  to  cross  altogether  or  gives  a  sterile  product,  whatever 
type  be  chosen  as  the  other  parent. 

Examples  of  this  kind  have  at  least  the  merit  that  they  lead 
to  more  precise  delimitations  of  the  problem.  We  are  confronted 
with  two  distinct  alternatives. 

I.  We  may  apply  the  term  Species  promiscuously  to  all 
distinct  forms.  If  we  do  so  it  must  be  clearly  understood  that 
we  cannot  even  rule  out  the  several  combinations  of  "  presences 
and  absences  "  represented  by  the  various  types  whether  wild  or 
domesticated.  For  we  may  feel  perfectly  assured  that  at  least 
all  the  arvense  and  all  the  sativum  types  yet  subjected  to  experi- 
mental tests  are  on  precisely  the  same  level  in  this  respect.  There 
is  no  distinction,  logical  or  physiological,  to  be  drawn  between 
them.  Some  contain  more  factors,  and  others  contain  fewer. 
In  some  the  re-combinations  have  been  brought  about  by  natural 
variation  or  crossing,  while  the  same  consequences  in  the  others 
have  resulted  from  man's  interference. 

u  In  a  paper  about  to  appear  in  Jour  Linn.  Soc.  Mr.  A.  W.  Sutton  identifies 
this  Palestine  pea  as  Pisum  humile  of  Boissier  and  Noe. 


STERILITY  OF  HYBRIDS  247 

2.  We  may  follow  the  conventions  of  systematists  and  dis- 
tinguish the  outstanding  or  conspicuous  forms  such  as  arvense, 
guadratum,  sativum  and  perhaps  a  few  more  as  species,  and  leave 
the  rest  unheeded.  If  this  course  is  followed  it  must  be  clearly 
understood  and  permitted  as  a  piece  of  pure  pragmatism,  deliber- 
ately adopted  for  the  convenience  of  cataloguers  and  collectors, 
without  regard  to  any  natural  fact  or  system  whatsoever. 

But  while  following  either  the  one  plan  or  the  other  we  shall 
be  still  awaiting  the  answer,  which  only  genetic  experiment 
can  provide,  to  the  question  whether  among  the  various  types 
there  are  some  which  differ  from  the  rest  in  a  peculiar  way: 
whether  by  having  groups  of  characters  linked  together  in 
especially  durable  combinations,  or  by  possessing  ingredients 
which  cause  greater  or  less  disturbance  in  the  processes  of  cell- 
division,  and  especially  in  the  processes  of  gametic  maturation, 
when  they  are  united  by  fertilisation  with  complementary 
ingredients. 

Before  any  but  the  vaguest  ideas  regarding  the  nature  and 
significance  of  inter-specific  sterility  can  be  formed,  a  vast 
amount  of  detailed  work  must  be  done.  Sterility  as  a  result  of 
crossing,  as  well  as  that  which  is  alleged  sometimes  to  arise  in 
consequence  of  changed  conditions,  is  at  best  a  negative  charac- 
teristic, and  there  are  endless  opportunities  for  mistake  and  mis- 
interpretation in  studying  features  of  this  kind.  No  one,  I 
suppose,  would  now  feel  any  great  confidence  in  most  of  the  data 
which  from  time  to  time  are  resuscitated  for  the  purpose  of  such 
discussions.  Even  the  best  collections  of  evidence,  such  as  those 
given  by  Darwin  in  Forms  of  Flowers,  cannot  be  regarded  as 
critical  when  judged  by  present-day  standards.  Nothing  short 
of  the  most  familiar  acquaintance  with  the  habitual  behaviour 
of  individuals,  and  of  strains  kept  under  constant  scrutiny  for 
several  years  would  enable  the  experimenter  to  form  reliable 
judgments  as  to  the  value  to  be  attached  to  observations  of 
this  class. 

The  admission  must,  however,  be  faced  that  nothing  in  recent 
work  materially  tends  to  diminish  the  surprise  which  has  always 
been  felt  at  the  absence  of  sterility  in  the  crosses  between  co- 


248  PROBLEMS  OF  GENETICS 

derivatives.  We  should  expect  such  groups  of  forms  to  behave 
like  the  Erophila  types,  and  frequently  to  produce  sterile  products 
on  crossing.  Whatever  be  the  explanation,  the  fact  remains 
that  such  evidence  is  wanting  almost  completely.  In  spite  of 
all  that  we  know  of  variability  nothing  readily  comparable  with 
the  power  to  produce  a  sterile  hybrid  on  crossing  with  a  near 
ally,  has  yet  been  observed  spontaneously  arising,  though  that 
characteristic  of  specificity  is  one  of  the  most  widely  distributed 
in  nature.  It  may  be  that  the  lacuna  in  our  evidence  is  due 
merely  to  want  of  attention  to  this  special  aspect  of  genetic 
inquiry,  and  on  the  whole  that  is  the  most  acceptable  view  which 
can  be  proposed.  But  seeing  that  naturalists  are  more  and 
more  driven  to  believe  the  domesticated  animals  and  plants  to  be 
poly-phyletic  in  origin — the  descendants,  that  is  to  say,  of  several 
wild  forms — the  difficulty  is  proportionately  greater  than  it  was 
formerly,  when  variation  spontaneously  occurring  was  regarded 
as  a  sufficient  account  of  their  diversity. 

CONCLUDING  REMARKS. 

The  many  converging  lines  of  evidence  point  so  clearly  to 
the  central  fact  of  the  origin  of  the  forms  of  life  by  an  evolutionary 
process  that  we  are  compelled  to  accept  this  deduction,  but  as 
to  almost  all  the  essential  features,  whether  of  cause  or  mode, 
by  which  specific  diversity  has  become  what  we  perceive  it  to 
be,  we  have  to  confess  an  ignorance  nearly  total.  The  trans- 
formation of  masses  of  population  by  imperceptible  steps  guided 
by  selection,  is,  as  most  of  us  now  see,  so  inapplicable  to  the 
facts,  whether  of  variation  or  of  specificity,  that  we  can  only 
marvel  both  at  the  want  of  penetration  displayed  by  the  advo- 
cates of  such  a  proposition,  and  at  the  forensic  skill  by  which  it 
was  made  to  appear  acceptable  even  for  a  time. 

In  place  of  this  doctrine  we  have  little  teaching  of  a  positive 
kind  to  offer.  We  have  direct  perception  that  new  forms  of  life 
may  arise  sporadically,  and  that  they  differ  from  their  progenitors 
quite  sufficiently  to  pass  for  species.  By  the  success  and  main- 
tenance of  such  sporadically  arising  forms,  moreover,  there  is 
no  reasonable  doubt  that  innumerable  strains,  whether  in  isola- 


CONCLUDING  REMARKS  249 

tion  or  in  community  with  their  co-derivatives,  have  as  a  fact 
arisen,  which  now  pass  in  the  lists  of  systematists  as  species.  For 
an  excellent  account  of  typical  illustrations  I  would  refer  the 
reader  to  the  book  lately  published  by  R.  E.  Lloyd14  on  the  rat- 
population  of  India.  The  observations  there  recorded  are  typical 
of  the  state  of  things  disclosed  whenever  the  variations  of  large 
numbers  of  individuals  are  closely  investigated,  whether  in 
domestication  or  in  natural  conditions. 

Guided  by  such  clues  we  may  get  a  good  way  into  the  prob- 
lem. We  see  the  origin  of  colourable  species  in  abundance. 
Then,  however,  doubt  arises  whether  though  these  new  forms 
are  as  good  species  as  many  which  are  accepted  as  such  by  even 
cautious  systematists,  there  may  not  be  a  stricter  physiological 
sense  in  which  the  term  species  can  be  consistently  used,  which 
would  exclude  the  whole  mass  of  these  petites  esp&ces. 

If  further  we  find  that  we  have,  with  certain  somewhat 
doubtful  exceptions,  never  seen  the  contemporary  origin  of  a 
dominant  factor,  or  of  inter-racial  sterility  between  indubitable 
co-derivatives,  it  needs  no  elaboration  of  argument  to  show  that 
the  root  of  the  matter  has  not  been  reached. 

Examination  of  the  inter-relations  of  unquestionably  distinct 
species  nearly  allied,  such  as  the  two  common  species  of  Lychnis, 
leads  to  the  same  disquieting  conclusion,  and  the  best  suggestion 
we  can  make  as  to  their  origin  is  that  conceivably  they  may  have 
arisen  as  two  re-combinations  of  factors  brought  together  by  the 
crossing  of  parent  species,  one  or  both  of  which  must  be  supposed 
to  be  lost. 

All  this  is,  as  need  hardly  be  said,  an  unsatisfying  conclusion. 
To  those  permanently  engaged  in  systematics  it  may  well  bring 
despair.  The  best  course  for  them  is  once  for  all  to  recognise 
that  whether  or  no  specific  distinction  may  prove  hereafter  to 
have  any  actual  physiological  meaning,  it  is  impossible  for  the 
systematist  with  the  means  at  his  disposal  to  form  a  judgment  of 
value  in  any  given  case.  Their  business  is  purely  that  of  the 
cataloguer,  and  beyond  that  they  cannot  go.  They  will  serve 
science  best  by  giving  names  freely  and  by  describing  everything 

"Lloyd,  R.  E.,  The  Growth  of  Groups  in  the  Animal  Kingdom,  London,  1912. 


250  PROBLEMS  OF  GENETICS 

to  which  their  successors  may  possibly  want  to  refer,  and  gen- 
erally by  subdividing  their  material  into  as  many  species  as 
they  can  induce  any  responsible  society  or  journal  to  publish. 
Between  Jordan  with  his  200  odd  species  for  Erophila,  and 
Grenier  and  Godron  with  one,  there  is  no  hesitation  possible. 
Jordan's  view,  as  he  again  and  again  declares  with  vehemence,  is 
at  least  a  view  of  natural  facts,  whereas  the  collective  species  is  a 
mere  abstraction,  convenient  indeed  for  librarians  and  beginners, 
but  an  insidious  misrepresentation  of  natural  truth,  perhaps 
more  than  any  other  the  source  of  the  plausible  fallacies  regarding 
evolution  that  have  so  long  obstructed  progress. 

Nevertheless  though  we  have  been  compelled  to  retreat  from 
the  speculative  position  to  which  scientific  opinion  had  rashly 
advanced,  the  prospect  of  permanent  progress  is  greatly  better 
than  it  was.  With  the  development  of  genetic  research  clear 
conceptions  have  at  length  been  formed  of  the  kind  of  knowledge 
required  and  of  the  methods  by  which  it  is  to  be  attained.  If 
we  no  longer  see  how  varieties  give  rise  to  species,  we  may  feel 
confident  that  a  minute  study  of  genetic  physiology  of  varieties 
and  species  is  the  necessary  beginning  of  any  critical  perception 
of  their  inter-relations.  It  is  little  more  than  a  century  since 
no  valid  distinction  between  a  mechanical  mixture  and  a  chemical 
combination  could  be  perceived,  and  in  regard  to  the  forms  of 
life  we  may  well  be  in  a  somewhat  similar  confusion. 

As  yet  the  genetic  behaviour  of  animals  and  plants  has  only 
been  sampled.  When  the  work  has  been  done  on  a  scale  so 
large  as  to  provide  generalisations,  we  may  be  in  a  position  to 
declare  whether  specific  difference  is  or  is  not  a  physiological 
reality. 


INDEX  OF  SUBJECTS 


PAGE 

Abraxa  grossulariata 105,  193 

Aceras  hircina,  local  variability. . .  123 

Achatinellidae,  local  forms  of 133 

Acquired  characters,  inheritance  of, 

1 88,  et  seq.,  217,  233 

Acronycta  psi,  melanic 138 

Adaptation,  problem  of 187,  234 

Agelaius,  local  forms 120 

Agrotis,  fixed  and  variable  species.  25 

Alkaptonuria 83 

Alpine    Plants,    growing   larger,   if 

protected 183 

Alpine  Varieties 165 

Alytes    obstetricans,     Kammerer's 

experiments  on 199.  210 

Amblystoma,  races  of 230 

Amphidasys  betularia.melanic  form, 

136,  138 

dimorphic  larvae 141 

Anodonta,  polymorphism  of 130 

Antirrhinum,  striped 57 

species-hybrids 99 

albinos no 

Apple,  will  not  cross  with  pear 239 

Arctia  caja,  effects  of  temperature.  192 

larval  variation  in 231 

Arctic  varieties 165 

Argynnis   paphia  and   valesina  in 

Italy 121 

Armadillo,  polyembryony 42 

Artistic  faculty 89 

Arum,  rights  and  lefts 57 

Auriculas,  short-styled  selected . . .  236 

Axis  of  symmetry  in  hand  and  foot  48 

Axolotl,  alleged  effect  of  conditions  230 

Azalea,  bud-sports 55 

Bacillus    anthracis,     unsegmented 

form 71 

Bacillus  prodigiosus,  variation  in. .  213 

Bacteria,  variation  in 212 

Bacterium  coli,  variation  in 214 

Baeolophus,  geographical  races  of . .  159 

Barley,  right  and  left-handed 58 

Basilarchia,  geographical  races  of. .  161 

Begonia  phyllomaniaca 50 

hybrids 51 

Bizarre  Carnation,  genetics  of 54 

Black,  as  a  variation  from  red ....  148 


PAGE 

Blackbird,  varying 150 

Black  Cock,  fixity  of 28 

Boarmia  repandata,  melanic  form .    136 

rhomboidaria 137,  139 

Botrytis  susceptibility  to 108 

Bovidae,  hybrid 242 

Brachydactyly 89,  95 

Bradypus,  vertebral  variation 68 

Bud-sports  geometrically  irregular  54-57 
Buffalo,  attempts  to  hybridize. . . .   242 

Bullfinch,  gynandromorph 45 

Bulimus  detritus,  local  variation  of  126 

Canary,  asymmetrical  markings  in  154 

Canidae,  hybrid 241 

Capsella 100 

Cardamine  pratensis 239 

Cat,  Polydactylism 53 

Carnation,    Picotees   and   bizarres 

compared 54,  58 

Cataract,  hereditary 89 

Certhiola,  melanic 142 

Chladni  figures 60 

Choloepus,  vertebral  variation  in . .     68 

local  variation  in 119 

Cinerarias,  self -sterility  in 238 

Cistudo,  local  variation  in 119 

Climatic  varieties 164 

Coccaceae,  variation  in 213 

Coenonympha     arcania,     climatic 

forms  of 179 

satyrion 180 

Coereba,  melanic 142 

Colaptes,  geographical  races,  147,  et  seq. 

chrysoides 154 

Colloids,  growth  in 65 

Colorado  beetles,  experiments  on. .   218 

Colour  blindness  in  twins 44 

Continuous  variation,  possible  ex- 
ample of 173 

Coracias,  geographical  races  of. ...    160 

Cotton,  genetics  of 98,  100 

Coupling no 

Crab,  extra  claws 74 

Crustacean  appendages  and  Serial 

Homology 63 

Crystals,  analogy  with 78 

Cyclopian  monsters,  artificial 50 

Daphnia,  changed  by  environment  216 


251 


252 


INDEX  OF  SUBJECTS 


Dasypus,  polyembryony 42 

Dianthoecia,    fixed    and    variable 

species 25 

Disease-resistance 87 

Division,  power  of,  a  fundamental 

attribute  of  living  things 38 

Genetics  of 46,  50 

Dogger  Bank,  large  varieties  on. . .  125 

Dogs,  hybrid 241 

Dominance,  nature  of 95 

Dominants,  origin  of  new. .  .88,  90,  95 

Double  monsters 42 

Draba,  experiments  with 242 

Drosophila 91 

Payne's  experiments  on 228 

Earthworm,  regeneration 77 

Elephant,  tusk  segmented 38 

Entelechy 80 

Environmental  treatment,  effects  of, 

188,  et  seq. 

Enzymes  and  genetic  factors 86 

Epilepsy,  inheritance  of  traumatic.    197 

Equidae,  sterility  of  hybrid 241 

Erophila,  experiments  with 242 

species 249 

Exacum,  right  and  left 57 

Euphonia  elegantissima,  local  forms  120 
Eupithecia    rectangulata,    melanic 

form 137 

Factors,  new 88 

loss  of 96 

Factorial  representation  of  varieties 

158,  165 

Falcons,  geographical  races 147 

Fasciation 49 

Ferments,  Boyle  on 54 

Finger-prints  of  twins 44 

Fixity  and  Variability  in  species. .      25 

Flax,  climatic  experiments 197 

Fowl,  Silky 84 

Leghorn 85,  90 

Dominant  white 94 

Wyandotte 97 

Rumpless 46 

Foxes,  incompatibility  with  dogs. .    241 

Free-martin 44 

Fringillidae,  sterility  of  hybrid ....    241 
Fundulus,  cyclopian 50 

Callus,  invariability  of  wild  species     13 
and  origin  of  poultry 90,  97 

Genitalia,  a  basis  for  classification 
in  insects 13 

Gentians,  climatic  experiments.  ...    197 

Geometrical  structure  and  differen- 
tiation   54,  56 


Geometrical    distinction     between 

germ-cells  and  somatic  cells 58 

Gladiolus,  right  and  left 57 

Gnophus  obscurata,  protective  col- 
ouring    141 

Goldfinch,  geographical  races 147 

Gonioctena  variabilis,  variation  in 

sexes  of 121 

Gouldian  Finch,  polymorphism,  148,  149 
Gracilaria  stigmatella,  experiments 

on 193 

Grantia,  large  varieties  of 125 

Ground-Squirrels,  local  forms  of. .  132 

Grouse,  red,  variation 29 

Guillemot,  Ringed 150 

Guinea-pig,    Brown-Sequard's    ex- 
periments on 198 

Gynandromorphs 45 

Heliconius  erato,  forms  of 122,  164 

Helix  lapicida,  local  variation  of . .  126 

striata 127 

Heripensis 127 

Caespitum 127 

trochoides 127 

nemoralis  and  hortensis 128 

Helminthophila,  geographical  races 

of 157 

Hemerophila  abruptaria,  melanic.  .  142 

Hepialus  humuli,  in  Shetland 119 

Heterostyle  plants 236 

Hieracium 9 

Himantopus 234 

Homoeosis 68 

Hybernia  progemmaria 139 

Hybrids,  sterility  of 233  et  seq. 

Incompatibility     between     certain 

allied  species 239 

Individual,     geometrical    indepen- 
dence of 58 

Inhibiting  Factors 95 

Intermediates,  nature  of 131,  135 

Isolation,  consequences  of 118 

Lacerta   muralis,    Kammerer's   ex- 
periments on 209 

fiumana 210 

Leptinotarsa,  Power's  experiments 

on 218 

Limbs,  extra,  in  pairs 72 

Limnaea,  sinistral 134 

Linaria  vulgaris,  self-sterility 239 

Loasa  fruits,  right  and  left 57 

Lobster,  extra  claws 76 

Locality,  variation  connected  with 

14,  118,  146,  et  seq.,  208 

Lumbricus,  regeneration 77 


INDEX  OF  SUBJECTS 


253 


Lychnis  dioica  and  vespertina,  in- 
ter-relations of 18 

macrocarpa,  possibly  a  common 
parent  of 19 

Machetes  pugnax,  polymorphism  of 

male 28 

Maize,    Blaringhem's    experiments 

on 229 

Maize,  cumulative  factors  in 116 

Malformations,  dominants,  arising 

de  novo 89 

Manx  Cat,  heredity 46 

Matthiola 84,  104,  113 

Melanic  varieties 135,  et  seq. 

Memory,  analogy  with  heredity. .  .    190 

Meristic  variation 69,  83,  86 

Mirabilis,  striped 57 

Models  of  segmentation 59,  60 

"Modes,"  Coutagne's  conception  of  126 
Modling,    peculiar   race    of   Pieris 

napi  at 178 

Mole,  albino 27,  28 

Mule,  Linnaeus  on 8 

Mutation,  Matthioli  on 4 

in  Mercurialis 5 

in  Kales 5 

alleged  in  bulbs 5 

Theory 97 

periods  of 114 

in  Bacteria 214 

Mutilation,  consequences  of 71 

alleged  effect  of,  on  offspring. . .    229 
Myxococcus,  variation  in 213 

Narwhal,  asymmetry  of  tusks. ...     44 

Nemesia  strumosa 91 

Neuration,  a  basis  for  classification     13 

Nicotiana,  sterility  of  hybrid 242 

Nightjars,  varying 150 

Noctuidae,  fixity  and  variability.  .      25 
Noctua,     polymorphic    and     fixed 

species 25 

Noctua  castanea,  local  forms  of . . .    122 

Nomenclature,  future  of 94,  245 

Notonecta,  variations  of 130 

Odontoptera     bidentata,     melanic 

form 137 

Oedipodidae,  protectively  coloured  140 

Oenothera,  new  dominant  in 92 

nibricalyx  and  rubrinervis 92,  95 

Lamarckiana 92,  101 

origin  of 102,  244 

has  bad  pollen-grains 102 

factorial  analysis  of 103 

pollen  and   egg-cells  genetically 
dissimilar 104 


Oenothera,   "twin  hybrids" 105 

laeta  and  velutina 105 

reciprocal  crosses  in 105,  et  seq. 

possible  coupling  in in 

dwarfs 112,  114 

"Triple  hybrids" 114 

alleged   variation  due  to  treat- 
ment    227 

Ophrys,  local  variability 125 

Orange,  polyembryony 45 

Osmotic  growth 65 

Overlapping  forms 146,  174 

Papilio,  geographical  races  of 162 

Papilio  turnus,  variation  of 144 

Pararge  egeria,  geographical  forms 

1 66  et  seq. 

Parthenogenesis 50 

Partula,  local  forms  of 133 

Passer  domesticus  and  montanus, 

distinctions 22 

Pea,  round  and  wrinkled 95 

Pear,  will  not  cross  with  apple 239 

Pelargonium,  variegated 55 

bud-sports 56 

Periodic  phenomena  in  structure. .      63 
Peronea,  fixed  and  variable  species     26 

"  Petites  especes  " 248 

Petunia,  double 104 

Phalanger  maculatus,  local  varia- 
tion    119 

Pheasant,  fixity  of 29 

Phigalia  pilosaria,  melanic. . .  .139,  140 
Phratora  vitellinae,  experiments  on  193 

Phyllotaxis 69 

Pied  varieties  common  in  Passer 
domesticus  unknown  in  Mon- 
tanus    23 

Pieris  napi  and  bryoniae. ...  174,  et  seq. 

Pig,  mule-footed 46 

Pigeon,  web-footed 46,  49 

Indian  Rock,  a  recessive  form ...     98 

Pigments,  nature  of 83 

Pisum  humile,   hybrids  with  culi- 
nary peas 244 

species 246 

Planar ian,  regeneration  of 71,  77 

Plotheia  frontalis,  polymorphic.  .26,  29 

Plusia,  fixity  and  variation  in 26 

Poephila  gouldiae,  variation  of .  148,  149 

Polarity  of  individual 44 

Polia  chi,  melanic 138 

Polyanthus,  short-styled  selected .  .    236 

Polydactylism  in  Cat 52,  53 

Polyembryony 45 

Potato,  variation  in 91 

Poultry,  evolution  of 90 

Primula  obconica 91 


254 


INDEX  OF  SUBJECTS 


Primula  sinensis,  flaked 57 

Leaf -shapes 70 

new  dominant  in 92 

sterility  in 236 

"Giants" 236 

Primula,  species-hybrids 242 

Protective  coloration 140 

Pyrrhulagra,  local  forms 120 

Python,  twin- vertebrae 60 

Quiscalus,  geographical  races  of . . .  156 

Rabbit,  Angora 46 

colours  of 93 

Incompatibility  with  hare 242 

Raimannia   odorata,    Macdougal's 

experiments  on 226 

Rats,  Variation  in 248 

Recessives,  origin  of 90 

Reciprocal  crosses,  giving  distinct 

results 105,  et  seq. 

Regeneration 70 

Repulsion no 

Reversal  on  Regeneration 77 

Rhamphocoelus,  geographical  forms, 

159,  184 
Rhinoptera,  variation  in  jaws  of . .     38 

Rhythm  in  repetition 69 

Ribs,  variation  of 68 

Rights  and  Lefts 57~58 

Ripples,  analogous  to  segments  60,  66, 67 

regeneration  of 79 

Rollers,  geographical  races  of 160 

Ruff,  polymorphism  of  male 28 

Salamandra,    maculosa    and    atra 

182,  199,  203 

spotted  and  striped 207 

geographical  variation  of 208 

Segmentation,  nature  of 63 

simulated  mechanically 64 

compared  with  rippling 65 

analogies  with 68 

Segmentation   of   normally   unseg- 

mented  structures 38 

Selection,  Natural,  an  insufficient 
cause   of    definiteness   of  types, 

17,  134,  142 

Sempervivum 250 

Serial  Homology,  the  true  nature  of, 

62,  66 

Setina,  Alpine  varieties 181 

Sex  of  Twins 44 

Sex-factors,  possible  coupling  of . . .   in 
Sexual  characters,  variation  in  1 19  et  seq. 

Siamese  twins 44 

Silky  Fowl 84,  85 


Simocephalus,  changed  by  environ- 
ment     218 

Sinistral  forms I33~4 

Situs  transversus 43 

Skate's  jaws,  variation  in 38 

Sloths,  vertebral  variation 68 

Species,  conceptions  of,  3, 94, 99,  240,  245 

allied,  distribution  of 185 

alternative  uses  of  the  term 245 

Specific  difference,  universality  of . .      12 
of   organisms   compared   with 

those  of  inorganic  materials,     15 
failure  of  theory  of  Selection  to 

explain 18,  134,  247 

Sphyropicus  varius. 149,  156 

Spilosoma  lubricipeda,  varieties  of  181 

Zatima,  Heligoland  form 181 

Spinal  nerves,  segmentation  of. ...     67 

Sporadic  variation 131,  134,  248 

Squashes,  polymorphism  of 100 

Staphylococcus  pyogenes,  variation 

in 213 

Sterility  of  hybrids,  in  general ....   233 

in  Lychnis  hybrids 20  et  seq. 

in  crossing  forms  of  Draba 243 

Significance  of 244 

Self 238 

Stilt 234 

Stocks 84,  104,  113 

Striped  varieties 57 

Substantive  variation 84 

Subtraction-stages 93 

Supernumerary  limbs 72-76 

Sweet  pea,  variation  of 91 

sterile  anthers  in 237 

Symmetry  compared  with  heredity  41 
Symmetry  of  body  approximate..  78 

Syndactyly 47 

in  foot 48 

Syntheticf ormulae,  in  nomenclature    94 

Taeniocampa,    fixed   and   variable 

species 25 

Tamias,  local  forms  of 132 

Tanagers,  geographical  races  of . .  .  159 

Teeth,  variation  in 67,  39 

Tephrosia  consortaria  and  con- 

sonaria 137,  139,  140 

Tephrosia  species,  separated  by 

season 119 

Terminal  members,  variation  of . . .  68 
Thais  rumina,  local  variation  in. .  .  27 
Tolerance,  persistence  of  diversity 

due  to 17,  134 

Tomato,  number  of  cells  in  fruit.  .  46 
Transitional  populations,  rarity  of,  165 

an  example 178 

Tropaeolum,  sterile  anthers  in.  ...  237 


INDEX  OF  SUBJECTS 


255 


Trypanosomes,  variation  in 215 

Tusk,  of  Elephant,  segmented ....  38 

of  Narwhal 44 

Twinning 4*.  44.  7* 

heredity  of 45 

in  organs 46 

Uria  troile,  variety  of 150 

Vanessa  urticae,  effects  of  tempera- 
ture   191 

Variation,  a  medley  of  phenomena,  14, 15 

sporadic 131,  134 

and  locality 118 

Causes  of  genetic ...  86,  87,  131,  212 

Substantive  and  meristic 83 


Veronica,  specific  difference  in. 16 

intermediates  between  species..  17 

Vertebrae,  division  in 60,  61 

homologies  of 66 

Vespa,  specific  difference  in 23 

Vortex,  living  organism  compared 

with 40 

Wave-motion  compared  with  repeti- 
tion of  parts 62,  67,  79 

Wheat,  cumulative  factors  in 116 

climatic  experiments  on 195 

Woodpecker 234 

Zebra,  pattern  of  stripes  compared 

with  ripples 38 


INDEX  OF  PERSONS 


PAGE 

Ackermann 242 

Agar 218 

Allen,  J.  A 132,  147.  *59 

Annandale 47 

Arrigoni  degli  Oddi 167 

Backhouse SO 

Baker,  G.  T 166 

Bangs,  Outram 120,  142,  155 

Barrett 26,  136, 167,  173,  178,  193 

Baur,  E 55.  99 

Baur,  G up 

Beneden,  van 75 

Bentham,  on  species  of  Veronica ..      16 

Lychnis 21 

Primula 22 

Bernadin 42 

Bishop,  L.  B 153.  157 

Blaringhem 229 

Bobart 5 

Boisduval 182 

Boissier 19 

Borradaile 74.  75 

Boulenger,  E.  G 208 

Boulenger,  G.  A 182,  207,  209 

Boyle 5.  54 

Brewster,  W 149.  150 

Britton 227 

Brown,  T.  Graham 198 

Brown-Sequard 197  et  seq. 

Bruant,  P 51 

Buffon 234 

Butler,  S 189,  190 

Buysson,  R.  du 24 

Candolle,  de 245 

Carpenter,  J.  H 172 

Chapman,  F.  M 148,  156,  157,  158 

Chapman,  T.  A 13,  167,  182,  231 

Church,  A.  H 69 

Cieslar 197 

Clark,  Austin 142,  144 

Cockayne,  E.  A 43 

Cockerell,  T.  D.  A 224 

Compton,  R.  H 50,  58,  227 

Cope 230 

Cory 142 

Correns 239 

Coutagne 125,  et  seq. 


Darwin,  on  Variation ; i,  2 

Systematics 10 

Selection 134,  139 

Heterostyle  plants 236,  237 

Darwin,  F 190 

Darwin,  Sir  G 41 

Davenport 46 

Davis,  H.  M 102 

Delcourt 130 

Deschange 181 

Dobell 215 

Doncaster 105,  121,  136 

Driesch 80,  81 

Duchartre 51 

East 91,  116 

Edwards,  W.  H 162 

Ehrlich 215 

Fellmer 215 

Field,  W.  L.  W 161 

Fischer,  E 192 

Fleck 171,  174 

Fletcher,  W.  H.  B 26,  181 

Foster,  Sir  N 39 

Galle 123 

Garrod 83 

Gates 92,  95,  102 

Gayner,  F 177 

Godron 249 

Gold,  E 196 

Goldschmidt 116 

Goodwin,  E 137 

Gortner 226 

Greene,  E.  L 8 

Gregory,  R.  P 92,  100,  236 

Grenier 249 

Grover 173 

Gruber 48 

Gulick 119,  133 

Hamling 142 

Hampson,  Sir  G 26 

Harris 142 

Hartlaub 182 

Herbst 42 

Heribert-Nilsson 1 16 

Hewett 182 

Honing 105 


256 


INDEX  OF  PERSONS 


257 


Hunter,  John 44 

Jakowatz 197 

Janet 24 

Jeans 41 

Jenkinson 4<> 

Jentink 120 

Johannsen 195 

Jordan 185,  242,  249 

Kammerer 199.  et  seq. 

Keeble 236 

Klebs 250 

Krancher 182 

Kiichenmeister 44 

Kudicke 215 

Lamarck 9 

Lang,  A 128 

Lawrence,  W.  N 142,  145 

Leake,  H.  Martin 98,  100 

Leavitt 185 

Lecoq 99 

Lederer 167 

Leduc 64,  65,  80 

Leydig 182 

Linden,  M.  von 192 

Linnaeus 6,  7,  8 

Lloyd,  R.  E 248 

Locard 130 

Lock,  R.  H 242,  244 

Loeb 42,  45.  So,  71,  77 

Lotsy 99 

Lowe,  P.  R 143 

Macdougal,  W.  T 102,  226 

Marchant 7 

Mathew 171 

Matthioli 4 

Mayer,  A.  G 133 

Mendel,  Rediscovery  of 2 

On  Fasciation 49 

Merrifield 169,  172 

Miller,  W.  D 120,  149 

Morgan 71,  77,  91,  198 

Moggridge 125 

Nathusius,  S 242 

Nettleship 44 

Newman,  H.  H 42 

Newsholme 48 

Nilsson-Ehle 116,  169 

Norman,  A.  M 125,  156 

Ober 142 

Oberthiir 168,  170,  193 

Oliver,  J 45 


Page,  H.  E 167,  180 

Patterson,  J.  T 42 

Payne,  F 278 

Pellew 236 

Poll 45 

Porritt. 136 

Poulton 141 

Powers,  J.  H 230 

Pringsheim,  H 213 

Przibram.  .  .72,  78,  178,  194,  197,  199 

Punnett no 

Ray 4,  5 

Raynor 105 

Ridgway 10,  120 

Roedelius 195 

Rolfe 20 

Rosen,  F 242 

Rosner 42 

Rowland-Brown,  H 167,  180 

Sargent 185 

Saunders,  E.  R 84,  104,  112 

Schima 177 

Schroder 193,  194 

Schiibeler 195 

Semon,  R 190  et  seq. 

Sharrock 5 

Shull 100 

Speyer,  A 166,  170,  181 

Spillman 47 

Standfuss 135,  181,  191 

Staples- Browne 49,  98 

Staudinger 170,  179 

Stockard 50,  71 

Sutton 236,  244 

Tornier 72 

Tower,  W.  L 218-226 

Trechmann 133 

Tugwell 181 

Tutt,   J.  W.     On  Definiteness  of 

Species 13 

On  Plusia  interrogationis 26 

On  Tephrosia 119 

On  N.  castanea 122 

On  Pararge  egeria 167  et  seq. 

Verity,  R 171,  177 

Vries,  H.  de 101-115,  222,  239 

Walker,  G 49 

Weir,  Jenner 119 

Weismann 176,  188 

Wendelstadt 215 

Werbitzki 215 

Werner 209 

Wettstein 197 


258 


INDEX  OF  PERSONS 


Wheeler,  G 168,  171 

Wheldale 83 

Wilder 44 

Wille 197 

Williams,  H 167,  172 

Windle,  B.  C.  A : . .     43 


Winslow 213 

Wolf,  F 213 

Woodforde 123 

Woltereck 215 

Zeijlstra 114 


CALIFORNIA 


